。
She sat down and crossed her legs.
Literal
She [topic-は] sit-and feet [object-を] crossed.
腰掛ける literally means 'to hang one's hips,' the Japanese way of saying 'sit down on a bench, stool, or chair' — distinct from 座る, which works for any sitting (including on the floor). The compound captures the physical motion of letting hips rest on a perch. 足を組む ('to cross one's legs') uses 組む ('to assemble, to interlock'), which also describes braiding, joining, or pairing. The te-form 腰掛けて chains the actions sequentially: 'sat, then crossed.'