Something that requires little mental effort or intelligence to perform or understand. The term is often applied to decisions which are straightforward or sometimes to people who appear to lack intelligence.
'No-brainer' is American in origin and was first used there in the 1950s. The earliest example that I've found of its use in the sense of 'requiring little mental effort' is in this 'The Berrys' cartoon, by Carl Grubert, which appeared in the Long Beach Independent, December 1959:
The first example of the term with the meaning of 'easily made decision' is from the Canadian newspaper The Lethbridge Herald, January 1968, in a report on an ice hockey game:
He'd break in on a goalie and the netminder would make one of those saves that our manager-coach, Sid Abel, calls "a no-brainer."
The application of 'no-brainer' to individuals came later, after the term was already well established; for example, this from Tom Alibrandi's 1979 novel Killshot:
"I'll unanimously be voted the no-brainer-of-the-month award."
See other phrases that were coined in the USA.