Words which are special in that they have no direct translation in other languages. “Word” is a stretchable term here, this might also be a set phrases here and not just a single word. Some pieces are from here.
あげおとり: When you look after a haircut worse then before.
ありがた迷惑: When someone does you a favour you had not asked for, and the result is causing you inconvenience - but you need to be grateful nontheless.
バックシャン: A woman that looks far better from behind than from the front
ぼけっと: Staring blankly out into space, without any thoughts
八方美人: Being ungenuinely nice to everyone out of fear of being disliked
過労死: death from to much work
賢者タイム: Timespan after the sexual act when a man is free of desire and can think clearly
金継ぎ: the art of repairing pottery with gold, the piece gains a history by doing that
恋の予感: The feeling, upon first meeting someone, that you will eventually fall in love.
口寂しい: The feeling of having to put something in ones mouth, i.e. to eat or to smoke tabaco. Lit: 'the mouth is lonely'.
教育まま: A mother who is eagerly pushing the child to study
森林浴: 'forest bathing', recovery by walking in the woods
辻斬（つじぎり）: Act of trying out a new sword on a random stranger
積ん読（つんどく）: Buying books without reading them
侘寂（わびさび）: world view to accept the imperfect nature of life
よこめし: the experience of having conversation in a foreign language
七転び八起き(ななころびやおき）: Falling down 7 times, getting up 8 times