一も二も無く[ichimonimonaku] unhesitatingly
一言二言[hitokotofutakoto] a word or two
一石二鳥[issekinichou] killing two birds with one stone
一二に及ばず[ichininioyobazu] right away
一姫二太郎[ichihimenitarou] It's good to have a girl first and then a boy

うり二つ[urifutatsu] as alike as two melons (two peas in a pod)
黄道十二宮[koudoujuunikyuu] twelve signs of the zodiac
二度[futatabi] again
最小二乗法[saishounijouhou] least-squares method
三十二分音符[sanjuunibuonpu] 32nd note

お歯黒[ohaguro] tooth blackening
暗黒期[ankokuki] eclipse period
暗黒時代[ankokujidai] Dark Ages
暗黒大陸[ankokutairiku] Dark Continent (i.e. Africa)
黒い[kuroi] black

黒い霧[kuroikiri] thick fog
黒くする[kurokusuru] to blacken
黒ずむ[kurozumu] to blacken
黒っぽい[kuroppoi] dark