random talk; speaking without thinking; offhand remarks
Refers to saying whatever comes to mind, often carelessly or without responsibility. Commonly used in the phrase 口任せに言う.
He said something off the top of his head without thinking.
You can't trust an answer given on a whim.
Compound of 口 (mouth) and 任せ (leaving something to), literally 'leaving it to one's mouth', i.e., speaking without forethought.