Grammar fix
a splitting headache
a very bad headache
A very painful, intense headache — the kind that makes it hard to think or focus.
collocationB1common
Hear it in context
Notice where the phrase sounds natural.
I've got a splitting headache — I can barely look at the screen.
A: Do you fancy coming out tonight?
B: I'd love to, but I've got a splitting headache.
Fix the pattern
One small grammar move makes it sound right.
Not: I have a splitting head.
Say: I've got a splitting headache.
'Splitting' goes with 'headache', not 'head' — it's a fixed pairing.