Refers to a clenched hand. Often used in the context of a fist bump or a punch, such as 拳固を固める (to clench one's fist) or 拳固で殴る (to punch with a fist).
He clenched his fist in anger.
The children fought playfully with their fists.
Kana spelling is common in everyday writing, especially when the kanji is not important.
A shortened form of 拳骨 “fist,” with loss of the final つ.