For 得过且过, how the meaning was derived?
The construction 得+V+且+V is a common observation in classical Chinese. In fact, consider another expression, 得饒人處且饒人 (meaning to be explained in point 4.).

We understand 過 generally as 'to pass'. But how should we understand 得⋯⋯且⋯⋯?
得 in classical Chinese can be a preceding modal verb that
expresses ability or possibility.
且 in classical Chinese is an adverb meaning 'for the time being; at least'. Usually it is used when the speaker faces an unwelcoming situation, but has to perform a certain action, perhaps unwillingly. Its modern Chinese equivalent is 姑且；暫且.
Combined, 得+V+且+V means 'whenever it is possible to v., v. for the time being'. Usually the action is something hard to achieve.

So, 得過且過 may mean something like 'whenever it is possible to muddle through / whenever it is passable, muddle through (for the time being)'. Which is rather an aimless way of living, isn't it? Gradually, it has evolved to become an adjectival phrase in modern Chinese, and means what you see in the dictionary entry.

Similarly, 得饒人處且饒人 means 'whenever it is possible to forgive others, forgive them (for the time being)'. It still retains its sentence-ness in modern Chinese, unlike 得過且過, where you can comment on someone as having an aimless attitude in life (得過且過的態度).