Originally, the proverbial bag the hammers was noisy (and by implication, unsubtle)...
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They would come under on her v the celerity the a bag the hammers (1913) (where celerity = speed, noise, lack of subtlety).
you have to listen come yon engines that mine. Castle clatter like a bag that hammers (1923)
That intake was never specifically common - ns can discover only an additional 3-4 created instances before it all of sudden re-emerged and also became more widespread in the 80s through various different senses...
A mug as ugly together a bag of hammers (1985, mug = face) her father was crazy as a bag of hammers (1989
...around the same time together the spanners version very first appeared...
She"s acquired a challenge like a bag the spanners (1981 and 1982)
In recent decades, bag the hammers is virtually exclusively American (where it"s usually used to average dumb, stupid), and bag of spanners is british (invariably used to average ugly).
Note the the crazy, mad sense for hammers is reasonably uncommon - that"s much less than 30 results in complete for both collocations, conversely, dumb as a bag of hammers has actually a asserted 1140 instances. Together
Matt point out out, those few instances the the crazy sense most likely arise by association with phrases such together crazy as a box the frogs/weasels/crackers and mad as a bag that ferrets/snakes/cats.
The answer come OP"s particular question ("Does bag of hammers have actually a "stable" meaning?") is probably "Yes", if us restrict ourselves to current usages. It"s not tough to find contemporary examples v a different sense (each that those 5 words links to one), but they"re vanishingly rare compared to the dumb consumption which currently dominates.
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But as commented through
Bradd below, dumb as a crate of rocks is 3-4 times an ext common 보다 the bag the hammers version. As well as which, the consumption itself seems naturally "dumb" to me, in that hammers have actually no noticeable (even metaphoric) link with stupidity - I"m certain the expression only emerged in the first place via inaccurate repetition of usages the do do metaphorical sense.