屁 is used literally (fart) and as a vulgar intensifier meaning 'not at all' or 'nonsense'. 放屁 (fàngpì) literally means 'to fart' but also means 'bullshit!'. 屁话 = nonsense talk.
grammar
In the pattern verb + 个屁, 屁 acts as a dismissive negator: 懂个屁 (know nothing), 有个屁用 (completely useless).
