Is 'to lay a hit/blow on' someone (as in cheap shot) a slang expression?
The Corpus of Contemporary American English has one record for lay a hit and none for lay a blow. Neither is found in the British National Corpus. Of course, the expression may occur in other forms, such as laid a hit, but it seems unlikely that it is at all common. However, in British English, at least, lay one on him means to hit someone. Given the circumstances in which it is likely to be used, it will rarely if at all be found in formal writing, other than as part of a dialogue in fiction, perhaps.