你好！ Chai， 孑 means :longly But we mostly don't speak like that---他孑轻轻对我笑一下 mostly we use as : 孑然一身 。 May I know where did you learned this word? From book?

单独，孤单

What are the contexts?

Most probably you mean to say, 他孑孑对我一微笑, (He conspicuously gave me a gentle/tiny/slight smile) or 他輕輕孑孑对我一笑. It can also be 他轻轻对我一孑笑, He gently gave me a lonely smile. These are all poetic writing and not used in ordinary speech.

Never saw the expression before, it would be a wrong sentence. 他孑轻轻对我笑一下 makes no sense.