The Chicago Manual of Style, 14th Edition contains the following (on the hyphenation or otherwise of compounds):
6.38: The trend in spelling compound words has been away from the use of hyphens; there seems to be a tendency to spell compounds solid . .
.
One could argue that the word 'solid' is being used as a predicative adjective here (as in the lake froze solid) – but the semantic analysis is indeterminate, in my view, between adjectival and adverbial usage of 'solid' in the CMoS usage.
AHD lists the adverb polyseme:
solid adv.
1. As a whole; unanimously: The committee voted solid for the challenger.2.. Without a break or opening; completely or continuously: The theater was booked solid for a month.
To my surprise, Collins doesn't list it, though 'booked solid' is very common in the UK.
What word-class would people plump for in the CMoS usage – and, more importantly, what verbs (other than obvious link-verbs) may 'solid', when not obviously accompanying a noun, follow?
Best Answer
The OED has no entry for solid as an adverb, but under its entry for solid as an adjective its definition of to book solid is ‘to sell all the tickets of (a theatre, cinema, etc.)’.
If solid cannot be an adverb, there seems to me to be no alternative to regarding it as a predicative adjective in the examples you give. But I’m not entirely confident about it.