Two strands of meaning: financial crime (
侵吞公款 'embezzle public funds',
侵吞资产 'misappropriate assets') and territorial aggression (
侵吞土地 'annex land'). Both share the underlying image of greedily 'swallowing' something that doesn't belong to you. Compare
贪污 (tānwū, 'graft / corruption') which focuses on bribe-style corruption, while
侵吞 stresses outright misappropriation.