The Full Monty.

[sandyRoe]

I've just read a post on FB suggesting the term, 'The full Monty' came from Field Marshall Montgomery's taste for a hearty breakfast.

We shouldn't believe all we read on-line.

I'm sure it came from the tailor, Montague Burton, who sold three piece suits.

What say you?

[ToraToraTora]

The Burton one is the one I always thought it was. The full monty being a full 3 piece suit.

[Buenchico]

Here's what the OED has to say about the etymology of the phrase:
https://postimg.cc/06mPf1Mb

[gramps85]

I was led to believe the saying came from soldiers, who were demobbed after WW11, receiving a 3 piece demob suit from Burtons, were said to be receiving the Full Monty.

[sandyRoe]

I think the tailoring one is more likely.  Apart from anything else, Monty didn't look like someone who was a trencherman.