work; labor / achievement; ability; performance; talent / salary; earnings; income

brain; brains / mind; intellect; understanding

(anatomy) the brain / (by extension, figuratively) one's brains, one's mental functioning

(archaic, anatomy) the brain / (archaic, by extension) the head

science / a field of study

work done for free, unpaid work, uncompensated work / work done in vain, work with no favorable end result

to work for free, to work without pay or other compensation / to work in vain, to work with no favorable end result

(archaic, grammar) any word that declines or conjugates / (archaic, grammar) more specifically, a verb

social science

(colloquial) current boyfriend