adverb
somehow; for some reason; without knowing why
Expresses that something happens or feels a certain way without a clear reason; often used when the speaker is puzzled or can't explain the cause.
何かしら胸がざわつく。
My heart feels uneasy for some reason.
今日は何かしら調子が悪い。
I'm not feeling well today, somehow.
adverb
something or other; some ... or other
Used when the speaker cannot specify or chooses not to specify a thing; often implies vagueness or uncertainty about the object.
彼は何かしら言い訳を考えている。
He's thinking up some excuse or other.
何かしら食べるものはある？
Is there something to eat, anything at all?
何か is the standard word for 'something'; 何かしら adds a nuance of vagueness or inability to specify.
何となく also means 'for some reason' but focuses on a vague feeling or intuition, while 何かしら can be used for both things and reasons.
Derived from 何か (nanika, 'something') + 知ら (shira, from 知る 'to know'), literally 'I don't know what'. The 知ら part fossilized into しら, and the whole became a fixed adverbial expression.