It is a Hokkien(a Chinese dialect) or Taiwanese word for female sexual organ, literally.
It also means a girl, a bastard, a dickhead, a shithole and a motherfucker, etc..(see example 1 below)
(not often)It may also be a negative supportive adjective to be placed in front of a subject noun, like "fucking shit" in english. However, this word is always for describing negative things, not exactly like "fucking" in english. In english, "fucking" can be added in front of a subject or noun to express "very much". But, ci mai never has this linguistic function. (see example 2 below)
That man has just stolen my wallet, ci mai !
That ci mai has just kicked me in my groin !
Man A: Sup dog? U look fucked. Wut's botherin u so much ?
Man B: u know who's mike, rite ? i caught that ci mai shacking with my girl on my bed last night !
Example 2 :
ci mai kia/gia/gaah = fucking kid
(most common example, and perhaps the only example in this category)