oblige: [13] To oblige someone is etymologically to ‘bind them to’ something with a promise. The word comes via Old French obliger from Latin obligāre, a compound verb formed from the prefix ob- ‘to’ and ligāre ‘tie’ (source of English liable, ligament, etc). By classical times its original literal sense had been extended figuratively to ‘make liable, put under an obligation’. The synonymous obligate [16] comes from its past participial stem, as does obligatory [15]. => liable, ligament, obligatory[oblige etymology, oblige origin, 英语词源]