What does "but" mean in "Life is but a dream"? [duplicate]
Solution 1:
I've always interpreted phrases of that sort having an entire word (nothing) elided out, as in:
Life is [nothing] but a dream
The popularity trend of the phrase "is nothing but a" versus "is but a" indicates the frequency of the poetic latter formulation is trending down towards that of the former, to a point about 20 years ago where they are nearly equal in occurrence.
Solution 2:
According to Free Dictionary, this usage is as an adverb, meaning "merely" or "only".
e.g. Hopes that lasted but a moment.