the.com/brainer

means A decision so obvious it requires no real thought — the 'brainer' in 'no-brainer,' something easy enough that your brain barely has to engage.

from Built backward from 'no-brainer,' an American coinage (mid-20th century) meaning a choice requiring no brain to solve. The word 'brainer' almost never stands alone — it only exists as the amputated remainder, surviving in jokes and deliberate backformations like 'that's a real brainer' used sarcastically. So its 'origin' is really the shadow cast by its host word.