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

bath mat

Basseterre (the capital of Saint Kitts and Nevis)

a bath towel

to hijack a bus

a largemouth bass

bath time

busfare

(formal) we, us