Can't complain is a cheerful way of saying life is going well enough. Oh, can't complain, well, can't complain, can't complain really.
It is the standard British answer to how are you, and it manages to be positive without claiming anything. Saying can't complain is a small refusal to make a fuss, which is the whole point of it.
1phrasecan't complainA cheerful way of saying life is going well enough.
“When the neighbour asked how he was, Dad smiled and said, "Oh, can't complain."”
can't complain is a reply.
A shortening of I cannot complain, with complain from Old French complaindre, to lament.
Short for I cannot complain.
Etymology from Wiktionary, CC BY-SA
Practised in Small Talk & Everyday Phrases