What is the meaning of 冒 in 感冒？
According to 《汉语大词典》, 感冒 means:


  
  犹感受。
  一种传染病。多因气候变化，人体抵抗力减弱时为病毒感染所致。
  清代官员请假的托辞。
  


The entries 2 and 3 seem to be somewhat related.
Two entries imply two different stories.
水巷孑蠻 talks about the 2nd one.
songyuanyao talks about the 3rd one.

The question talks about disease, so I focus on this.

From 《汉语大词典》, the 18th meaning of 冒:


  
  中医病症名。病者神智不清。
  ●《素问•玉机真藏论》：「﹝春脉﹞太过则令人善怒，忽忽眩冒而巅疾。」
  ●《医宗金鉴•张仲景〈伤寒论·太阳病下〉》：「其人因致冒，冒家汗出自愈。」集注引程知曰：「冒者，神识不清，如有物为之冒蒙也。」
  


《汉语大词典》列出來源而未解釋，不易了解。
《汉语大词典》 lists the sources without explanation, so it's not easy to understand.

東漢 張仲景（公元 150－219 年）《傷寒論》的原文如下：
The original content in 《傷寒論》 is as follows.


  以此表裏俱虛，其人因致冒，冒家汗出自愈。


先看「冒」的意思。(The meaning of 冒.)

【一】《汉语大词典》引用 程知《傷寒經注》：
《汉语大词典》quotes from 程知《傷寒經注》：


  冒者，神识不清，如有物为之冒蒙也。


冒 means coma, like being covered by something.

此句有「冒蒙」一詞。
「冒」與「蒙」是同義字，都是「覆蓋」的意思。
程知 用「覆蓋」來形容病人的「昏迷」。
這表示「覆蓋」是「冒」的原義，而「昏迷」是其引申義。
There is a phrase 冒蒙 in the above sentence.
冒 and 蒙 are synonyms, which means "to cover".
程知 used "cover" to describe a patient's "coma".
It means that "cover" is the original meaning of 冒, and "coma" is its extended meaning.

【二】此外，王丙《傷寒論注》的解釋原文(第143條)如下：
In addition, 王丙《傷寒論注》explained it as follows.


  以此表裏俱虛，因虛其人致冒，理必然也。冒家者，謂凡因病而昏冒者也。然冒家或有汗出自愈，其所以然者，非表裏俱虛，乃邪正皆衰，表裏自和故也。


此注有「昏冒」一詞。
再查《汉语大词典》的昏冒。
This remark has the term, 昏冒.
《汉语大词典》explains it as follows.


  昏冒
  昏迷不醒。
  《医宗金鉴•杂病心法要诀·三化汤》：「三化气实风中府，昏冒闭满小承羌。」注：「昏冒，谓神昏不知人也。」
  《医宗金鉴•运气要诀·运气为病歌》「诸痛痒疮属心火」注：「心藏神，热乘于心，则神不明，故昏冒不省人事也。」


昏迷不醒 means "unconsciousness" or "to lose consciousness".

所以，到這裡，冒 ＝ 昏冒 ＝ 昏迷不醒。這是較嚴重的狀況。
So, up to here, 冒 ＝ 昏冒 ＝ unconsciousness. This is a more serious situation.

【三】我們來看 金代名醫 成無己 的解釋。
Let's take a look the explanation of a famous doctor 成無己 in the 金 dynasty.


  冒者鬰也。


查《汉语大词典》對鬰冒的解釋：
《汉语大词典》explains 鬰冒 as follows.


  中医指头晕目眩或昏迷的症状。


TCM refers to the symptom of dizziness or coma.

由此可知，「冒」也包括輕微的狀況，如：頭暈目眩。
According to this, 冒 also includes mild situations, such as dizziness.

【四】小結論

從以上的解釋，我們可以知道「冒」的原義是「覆蓋」，而其引申義是從輕微的頭暈到嚴重的昏迷。
From the explanations above, we can see that the original meaning of 冒 is "cover", and whose extended meanings range from the mild dizziness to the serious coma.

再看「致冒」的意思。(The meaning of 致冒.)

「致」是導致、引起、致使的意思，這應該沒有問題。
「冒」可以是「冒疾」或「冒癥」。
所以，「致冒」可以是「導致冒疾」或「引起冒癥」。
致 means "to lead, to arouse, to cause". There is no problem here, I think.
冒 may be a disease or symptoms.
Therefore, 致冒 may mean "to lead 冒, the disease" or "to arouse 冒, the symptoms".

《汉语大词典》說「冒」是「中医病症名」。
《汉语大词典》 states that 冒 is a name of 病症 in TCM。

問題就出在這「症」字。
若用正體字，「症」和「癥」分得很清楚，前者是「疾病」，後者是「病狀」。
但簡化後，變成同一字，就容易混淆了。
The problem lies in the word, 症.
If using the traditional characters, 症 and 癥 are clearly different.
症 means a disease, and 癥 the symptom.
However, they become the same word in simplified Chinese.
It is easy to confuse.

我猜，在大陸，「病症」大多是指「疾病」。
所以，「冒」會被解釋為「病名」。
I guess, in Mainland, 病症 mostly means disease.
Therefore, 冒 is interpreted as the name of a disease.

但是，解釋為「病癥」可能會更合適。
也就是說，「感冒」是「感得冒癥的病」。
而這「冒癥」包括從輕微的頭暈到嚴重的昏迷。
換句話說，「感冒」是借用「冒」的癥狀而得的病名。
感冒的時候，頭會有一種暈悶沈重的感覺。
這種現象就是輕微的「冒」。
But it may be more appropriate to interpret it as the name of symptoms here.
That is, 感冒 means to get the disease which has 冒, the symptoms.
The symptoms range from mild dizziness to serious coma.
In other words, 感冒 is the name of a disease that borrows 冒.
When we catch a cold, the head will have a dizzy and heavy feeling.
This phenomenon is exactly a slight 冒。

從字面上來看，「致冒」類似「感冒」，差在主動和被動。例如：
Literally, 致冒 is similar to 感冒, and the difference is in active or passive. For example,

（致冒）天冷導致冒疾/冒癥。
The cold weather leads to 冒疾/冒癥.

（感冒）某人因天冷感得冒疾/冒癥。
Someone gets 冒疾/冒癥 because of the cold weather.

當然，這裡的「冒」不單指「昏迷」。
Of course, 冒 here doesn't just mean coma.

另一個差別是：「感冒」現在已是一專有詞彙，而「致冒」不是。
Another difference is that 感冒 is a proper term now. 致冒 is not.

結論(Conclusion)

「感冒」是借用「冒」的癥狀而得的病名。
「冒」包括從輕微的頭暈到嚴重的昏迷。
感冒 is the name of a disease that borrows 冒, the symptoms.
The symptoms range from slight dizziness to serious coma.

So, the question: What is the meaning of 冒 in 感冒？
原問：「感冒」中的「冒」是什麼意思？

我認為：雖然「冒」的原義是「覆蓋」，但在這裡是指「感冒的一些癥狀」，這可以是頭暈甚或昏迷。
My answer is some symptoms of cold, which may be dizziness or even coma, although its original meaning is "to cover" or "covering".



P.S. 題外話：

「冒」有很多意思。
在 張仲景《傷寒論》中的「冒」也不只一個意思。譬如：


  其人叉手自冒心，心下悸，欲得按者。


病人心悸，覺得心跳得很快，直覺地用手蓋住左胸，也希望能有幫忙按壓的人。

這裡的「冒」是動詞，只有其原義「覆蓋」的意思，不能用上述的引申義來解釋。

冒 has many meanings.
The 冒 in 《傷寒論》 also has more than one meaning. For example,


  其人叉手自冒心，心下悸，欲得按者。


The patient gets palpitations, fells the heart beating quickly, instinctively covers the left chest with hands, and expects a person who can help press.

冒, a verb here, means "to cover", only the original meaning, and cannot be explained by the above-mentioned extended meaning.