Tactlessness is the noun — the quality of being tactless, or a particular act of being tactless. It works two ways, and students should know both. Uncountable, it names the trait itself: 'His tactlessness has cost him a lot of friends.' 'I was shocked by the tactlessness of the question.' Countable, though rarer, it names one specific clumsy thing that was said: 'another of his famous tactlessnesses' — but honestly, most speakers avoid that plural and say 'another tactless remark' instead, so I would not go looking for chances to use it. The word carries the same gentle judgement as the adjective: it points at a lack of skill in handling people, not at bad intentions. Because it is a long, heavy noun, it belongs mostly to writing and to careful, formal speech — a complaint letter, a book review, a manager's report. In ordinary talk, people usually turn it back into the adjective and say 'that was so tactless'. Its opposite is 'tact' itself, which is why the pair is a little lopsided: we have a short simple word for having the skill (tact) and a long clumsy one for lacking it (tactlessness), which is quietly funny given what the word means.

不得體、不懂拿捏分寸（名詞）；口語會說「白目」的那種狀態。中文很少用名詞形式，多半改成形容說法：「他講話很不得體」。