Phrase used when advising somebody of facts /an issue /and explanation (etc) using the most simplest of words in order that they may understand.

Often associated with 'full and frank conversation' the phrase 'words of one syllable or less' is actually gramatically incorrect in its use of 'or less' (essentially 'or less' makes no sense whereas 'or fewer' would be correct) but nicely conveys the situation.

Believed to have originated with the staff of Britain's railways in Southern England.
"What do you mean Bert has done it wrong again?

I'd better go and explain how to do it in words of one syllable or less"
by Clansman July 12, 2009
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