。
She glared at the rude salesman.
Literal
She [topic-は] rude salesman [object-を] glared-at.
にらみつける is a compound verb: にらむ (to glare) + つける (attaching intensity) — an intensified, sustained glare directed at someone. 失礼な (rude) modifies セールスマン (salesman, from English). The を marks the target of the glare. The compound ~つける adds force — this isn't a passing dirty look, it's a deliberate, piercing stare.