なんだか・何だかnandaka[1]atamadaka (head-high)

1. adverb(a) little; somewhat; somehow
   なんだかすごく辛つらいの。↗

   Somehow it's been a rough day.

• SynonymなんとなくNeutralAdverb for acting or feeling vaguely, without a clear reason (「なんとなく分かる」); 「何だか」 is more colloquial and often attaches to feelings.
• SynonymなぜかNeutralAdverb meaning 'for no known reason' (「なぜか涙が出る」); stresses that no reason can be found, and is common in speech.
• Related何かしらなにかしらNeutralAdverb meaning 'for no clear reason, somehow' (「何かしら不安だ」); it can also mean 'something or other', a use 「何だか」 does not have.
• RelatedどういうわけかNeutralSet phrase meaning 'for some reason' (「どういう訳か眠れない」); the emphasis is on the cause being unknown.

Use 「何だか」 for feelings you cannot explain (「何だか寂しい」「何だか変だ」); it is colloquial. For a vague, half-conscious sense pick 「何となく」; when the point is simply that the reason is unknown, use 「なぜか」 or 「どういう訳か」.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

1. 明確めいかくな理由りゆうが無ないさま。
2. 何なんと言いっていいか分わからず当惑とうわくするときの言葉ことば。