scratch-one-s-head

Verb

(third-person singular simple present scratches one's head, present participle scratching one's head, simple past and past participle scratched one's head)

(idiomatic) To puzzle, ponder, or wonder about something.
Faced with a box full of hundreds of small parts and forty pages worth of assembly instructions, he could do little more than stand there and scratch his head.

Origin

scratch + one's + head

English Wiktionary. Available under CC-BY-SA license.