敲竹杠 is a colloquial four-character idiom meaning 'to overcharge, fleece, or blackmail someone' — taking advantage of someone's vulnerable situation to demand money. It originates from an old story about smuggling silver inside bamboo poles; the literal image is 'striking the bamboo pole' (to discover hidden goods). As a separable verb (离合词), you can insert elements between 敲 and 竹杠: 敲了他的竹杠 ('fleeced him'), 敲了一大笔竹杠 ('charged a huge amount'). The term has a playful, informal tone — not as harsh as the English 'blackmail' in legal contexts.