idioms - Why don't we pluralize "foot" in measurements?

For example, to answer the question, "How tall are you?" valid answers include:

• Five feet.


• Five foot three.


• Five feet, three inches.

Why the discrepancy between feet and foot, seemingly only in the second case.

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This question is inspired by this question: "Forty foot" or "forty feet"?

edit: I do not believe the answer to this is related to the other question. The explanation for the other question is because of how adjective modifiers work. My question is a very different case, unrelated to adjectives. My observation is that I am asking about a particular exception case which applies only when "foot" is followed by a number which is assumed to be inches. That's extremely specific, and I doubt that the etymology has any relation to why we leave adjective modifiers singular.

Answer

I think it's "idiomatic by association".

(this NGram should be enough to at least prove a trend)

There's long-standing 'idiomatic' use of the singular noun-form in 'adjectival' compounds such as toothbrush, ten-foot bargepole, four-wheel drive, etc. The answer to "how tall are you?" isn't really a noun, and it isn't a verb. It's closest 'basic' linguistic element is in fact an adjective (describing your height). People sense this, so over the decades they've simply shown an increasing tendency to apply the same 'singularisation' rule they've always been used to in related contexts.

OP correctly identifies a tendency to use the older pluralised form with more 'precise' measurements. One reason may be that people speak more carefully knowing they've got the longer utterance to come. Without conciously thinking, they just override the 'idiomatic' tendency in favour of the 'older, but perhaps more accurate' plural. The ones who do that are decreasing all the time, but obviously most of us would avoid pluralising the explicitly-named inches if we hadn't already pluralised feet.

It's the same with UK x pounds y pence (and US X dollars y cents, maybe?), where you often hear the first unit singularised and the second omitted. I suspect there's an increasing tendency to omit both units just to avoid the awkwardness of possible mixed plurality.