noun
blow received by a bystander; getting dragged into someone else's fight
Used in the idiom 側杖を食う (そばづえをくう), meaning to suffer harm or get involved in trouble not originally one's own. Often refers to being caught in the crossfire of a dispute or conflict.
夫婦喧嘩の側杖を食って、子供が泣き出した。
Caught in the crossfire of the couple's quarrel, the child started crying.
会社の派閥争いで、関係ない社員が側杖を食うこともある。
In company factional disputes, uninvolved employees sometimes get dragged in.
とばっちり is a more colloquial term for being caught up in trouble or suffering collateral damage, often used in everyday speech. 側杖 is more literary and typically appears in the set phrase 側杖を食う.
Compound of そば (side, nearby) and 杖 (stick, cane). The image is of a blow from a cane meant for someone else hitting a bystander, extended metaphorically to any collateral damage.