labouring oar
(US laboring oar)
noun
An oar that works or is worked, especially with difficulty or great effort; (hence figurative ) the most difficult or demanding part of an enterprise (now chiefly US ). Frequently in to pull (also tug, ply) the labouring oar (and variants).
Origin
Early 17th century; earliest use found in Michael Drayton (1536–1631), poet.