Something unbidden arrives without anyone asking for it, inviting it, or expecting it. The old verb 'bid' here means to ask or command someone to do something, so unbidden is 'not asked for'. It splits into two uses that feel quite different. The first is about people and physical things: a guest who turns up unbidden was not invited; a dog that comes unbidden came without being called. This use sounds formal and a bit old-fashioned — in speech you would say 'without being asked' or 'uninvited'. The second use is the one you will meet far more in modern writing, and it is about what happens inside the mind. Thoughts, memories, tears, images come unbidden: they arrive on their own, and you did not summon them and cannot stop them. 'Her name came unbidden into his head.' 'Tears rose unbidden.' That is the heart of the word today — it names the strange feeling of your own mind acting without your permission, which is why writers love it. Grammatically it usually sits after the verb, describing how something arrived, rather than in front of a noun. 'The memory came unbidden' is far more natural than 'an unbidden memory', although the second is possible and does get written. It has no comparative: nothing is 'more unbidden'.