fag (3) — ODE

fag3 /faɡ

▸ noun

1 [in singular] informal mainly British English a tiring or unwelcome task:

it's too much of a fag to drive all the way there and back again.

2 British English a junior pupil at a public school who does minor chores for a senior pupil:

a fag at school who has suffered a well-earned beating.

▸ verb

( fags, fagging, fagged)

[no object]

1 informal mainly British English work hard, especially at a tedious task:

he didn't have to fag away in a lab to get the right answer.

2 British English (of a public-school pupil) do minor chores for a senior pupil:

the lower boys in each house fagged for members of the Library.

– ORIGIN mid 16th century (as a verb in the sense ‘grow weary’): of unknown origin. Compare with flag4.