。
She's angry at the naughty boy.
Literal
She [topic-は] naughty-boy [at-に] belly [object-を] is-standing.
Contains the idiom 腹を立てる ('to get angry' — literally 'to stand one's belly'), reflecting the Japanese cultural association of the belly (腹) with inner emotions. 腕白小僧 (わんぱくこぞう, 'naughty boy/brat') uses ateji — the kanji 腕 ('arm') and 白 ('white') are assigned purely for their sound, with no connection to the word's meaning.