Pronunciation and etymology of 啱
So it is a general rule of thumb that any Cantonese syllable with the voiced initial consonants ng-, n-, l-, m- plus tones 1/2/3 is not "Classical", as it violates the defining voicing-based tone split of the four tones that happened in the transition from Middle Chinese to Modern Chinese. 

This means that 啱 is either not derived from Middle Chinese (so from a Tai-Kadai or an Austroasiatic or other non-Sinitic source), or it has gone through a more complex history that means it's only indirectly traceable to a Old or Middle Chinese lexeme (it thus might have a regular Old/Middle Chinese complement, which is often considered its 本字).

In the world of Chinese word etymology, things have very seldom been certain. A lack of good corpus data and earliest attestations for these dialectal characters is a major reason for the current state of affairs. Not much can be said "definitively" at all, though we do know it is not in the Kangxi Dictionary.

The theories on English Wiktionary seem possible, perhaps even reasonable (though unsourced at this point; I have found a source that points to the connection with 諳 deriving from a certain Mr 楊子靜, perhaps through his 廣州話分韻詞林), but that is all that can be said about them really. 

The other colloquial word that comes to mind with ng- with tones 1/2/3 is 奀, although that has a variant with the null initial too.