Is ‘Yes-ish’ a perfect alternative to Yes, or is it 'Yes ‘on condition’? Is it received English?

Solution 1:

'ish' is added to some words to denote 'sort of'. For example:

He was tall-ish

Means he was tall, but not extremely so. Taller than average, but not basketball-player tall.

I'm tired-ish

Means you're a little tired but not totally 'knackered' (a bit of British slang there).

Generally '-ish' is considered informal; slightly slang-ish!

You perhaps wouldn't use it in formal situations like a business meeting. It's not considered rude, it's just something you'd use in more relaxed settings.

This reminds me of a joke by a British comedian called Jimmy Carr

Catholics have a confirmation where someone says, "You are definitely Catholic". Jews are never sure: they say "Well, I'm Jew-ish".

Here the comedian has made a 'play on words' and used the 'sort of' meaning of -ish as opposed to that seen in Jewish, English etc.

Solution 2:

The definition for -ish is

-ish
suffix forming adjectives:
1. (from nouns) having the qualities or characteristics of: apish, girlish.
• of nationality or religious or ethnic group: Swedish , Amish, Flemish.
2. (from adjectives) somewhat: yellowish.
• informal denoting an approximate age or time of day: sixish.

I think that yes-ish can be understood as somewhat yes, even yes is not an adjective and -ish is not used to form an adjective.

Solution 3:

I don't know if I have ever heard the term "Yes-ish" in actual conversation, and I've certainly never read it. That being said, as a native (American) English speaker, I would know what someone meant if they said "Yes-ish".

"-ish" can be added intelligibly to a number of words (primarily adjectives), but it is not generally done. When I choose to use "-ish" in conversation, it's generally an attempt at humor. In most cases, you would just use a modifier, e.g., "I am pretty sure" instead of "I am sure-ish".