sound judgment; sane decision; act of a clear mind
Almost always used in negative constructions (e.g., 正気の沙汰ではない) to express that something is crazy, unthinkable, or beyond reason.
Buying it at this price is sheer madness.
His behavior doesn't seem like the act of a sane person.
正気 means 'sanity' or 'consciousness' on its own, while 正気の沙汰 is a set phrase meaning a sane or reasonable act, almost always used negatively.
Compound of 正気 (sanity, sound mind) + の (possessive particle) + 沙汰 (act, deed, affair). The phrase literally means 'an act of a sane mind' and is used idiomatically in negative contexts.