As in “It travels faster than sound at least.”
After considering the alternative at the very least, I'm thinking at is a preposition, and least is — well, stumping me. Can we have it be as normal, a superlative adjective simply standing in place of the noun phrase a preposition calls for? Or is it somehow a noun, or do prepositions not always require noun phrases, or do I just need to count *at least— as two words that somehow form an adverb?
The last option would be difficult, as I'm doing some part-of-speech tagging for a corpus I'm building. CLAWS web service auto-tags this phrase as
at_AV0 least_AV0, which would indicate both words are adverbs, but that seems very wrong, unless that's what they mean by it. But then I don't know how to distinguish tags indicating “these two words are one adverb” from “these are two adverbs”.