To unbend is to loosen up and stop being so stiff and formal. Picture a person held rigid — back straight, face careful, every word chosen — and then something lets that stiffness out of them. That is unbending. It usually needs a reason: a drink, an old friend arriving, the boss finally leaving the room, a holiday. 'After the second glass of wine he unbent a little and started telling stories.' Notice that the word describes a change, a moment of softening, not a permanent personality. And notice 'a little' — this word very often comes with 'a little', 'slightly', 'enough to…', because the people it gets used about are stiff by nature and only ever thaw partway. It sounds a bit formal and old-fashioned itself, which is a nice joke about a word that means relaxing. In everyday speech people say 'loosen up', 'lighten up', 'relax', or 'let his hair down'. The word is also literal in a plainer way: to unbend something bent is to straighten it — a wire, a paperclip, your knees after kneeling too long. That literal use is where the relaxing sense comes from, and it is completely ordinary English. It is an irregular verb, following bend: unbend, unbent, unbent.

（人）放鬆下來、卸下拘謹；（東西）弄直、扳直