expression, godan verb (-ku)
scratch one's head
Often done when embarrassed, shy, or puzzled. The gesture is a common body-language sign of awkwardness or thinking.
彼は照れくさそうに頭を掻いた。
He scratched his head, looking embarrassed.
質問に答えられず、頭をかくばかりだった。
Unable to answer the question, I could only scratch my head.