咋 is very colloquial and common in northern Chinese speech, where it replaces 怎么 in casual conversation.
usage
咋 can mean both how (咋办, how to deal with it) and why (你咋来了, why did you come). Two rarer readings exist: zhā in 咋呼 (to bluster; to make a fuss), and a literary zé meaning to bite. Everyday speech only uses zǎ.
Left mouth radical, the indexing component. Marks 咋 as a spoken-language word, a colloquial question word used in casual northern speech rather than written formality. The same radical anchors 啊, 呢, 吗, 呀, the everyday speech-particle family.
Right 乍 supplies the sound (zhà to zǎ, the retroflex flattening in the colloquial reading). Phonetic family: 炸 (explode), 榨 (squeeze), 诈 (deceive). The flavor of suddenness fits the how-come surprise that 咋 often expresses.
