means To strike sharply with an open hand or flat object, or the loud sound such a hit makes — also a noisy kiss, and slang for heroin.

from Likely imitative in origin — the word itself mimics the sharp 'smack' sound of a slap or a kiss, much like 'crack' or 'whack.' It's related to older Germanic and Low German forms (compare Dutch 'smakken,' to strike or fling down), all echoing that abrupt percussive noise. The heroin sense is a separate, much later 20th-century slang twist, possibly from Yiddish 'schmeck' meaning 'a sniff' or 'a taste,' which drifted into the drug world and got reshaped to sound like the familiar 'smack.'