1.
straight man role; retort, comeback
In Japanese comedy (especially manzai), the role of the person who corrects or retorts to the boke's (funny man's) absurd statements. By extension, also refers to the act of making such retorts in everyday conversation.
ツッコミ comes from manzai (漫才) comedy, where a duo consists of a ボケ (funny man who says absurd things) and a ツッコミ (straight man who corrects them).
Common expressions
The verb form ツッコむ means to make a retort. In casual conversation, ツッコむ and ツッコミ are used even outside of comedy contexts when someone points out flaws or absurdities in what others say.