A question of apostrophological significance...?

It is definately In Layman's Terms!

The apostrophe is a relic of the time when in the writing of the language a feature of it called the "weak or neutral e" disappeared.

The weak "e" was added to a word to show possession or, if you like, the genitive case: mannes right= the right of a man.

The "e" had the sound of "ugh" but much fainter. It was decided to remove it but it would leave a gap; the "e" was replaced with a floating comma, called an apostrophe. This practice occurs in any word where part has been missed in pronunciation, e.g. hasn't , the apostrophe occurs where the letter is missing.

in layman's terms = in the terms of A layman.

in laymen's terms = in the terms of laymen. In this case the apostrophe occurs before the final s because "men" is plural.

soldiers' songs= the songs of soldiers. Here the apostrophe comes after the s because that is the end of the plural word.

in laymans' terms means nothing at all; neither does the third example which would be starred in a grammar book to show that it was grammatically impossible.

So, apostrophe then s after a singular noun or a noun which

is plural but does not have an s. There are only a few of these latter and they are among the oldest words that we possess.

The apostrophe comes after plurals ending in s.

These problems only occur when writing the language.

Think about the meaning and the answer becomes clearer.

An apostrophe is an indication that something has been shortened. "It's" stands in for "it is", "I've" for "I have", and so on. The apostrophe goes at the end of the complete word, dividing it from the shortened section.

What you are trying to say is "in terms that could be understood by a layman". So "in layman's terms". It's the same for "the bag belonging to my mother" - "my mother's bag."

I hope that helps.

The apostrophe in this case does not indicate that any letters have been removed - it is to show possession.

The Elephants' feet (The feet of more than one elephant) (Elephants ' .)

The Elephant's feet (The feet of one elephant) (Elephant ' s)

See where the apostrophe goes?

laymens''ss's is not anything that I have seen before - forget it!

Words that end in "S" can be done a couple of ways...

For example .... The car belonging to the Jones family could be...

the Jones' car or the Joneses car.(I prefer the 1st way)

Your confusion doesn't seem to be about where the apostrophe goes so much as whether it's singular or plural, think about it in contrast to similar expressions:

teacher talk; geek talk; business speak; etc.

They're all singular. So, what you're looking for is:

in layman's terms

bonus: it's more politically correct to say, "in layperson's terms"

If it's one layman it must be "In layman's terms."

If more than one it's "In laymen's terms."

This is because the plural of man is men, not mans.

If you wanted to say "The bone of the dog" it would be..

"The dog's bone."

"The bones of the dogs" would be

"The dogs' bones."

It's quite easy really.

The apostrophe is used to denote belonging.

As in, these are the terms of the man:

man's terms.

If it were a plural, it would be:

mans' terms.

If it's a group, it's treated as a singular - as it is a singular group:

men's terms.