grammar - When an -i form (連用形{れんようけい}) of a verb seems to be a suffix rather than a prefix?

In a previous question about コーヒー割り I learned that 割り is a form of the verb 割{わ}る "to dilute".

Now I'm trying to understand the grammatical process by which this 割り form of 割る can be added to nouns such as 水 and コーヒー.

I've learned that 割り is the -i form, conjunctive, continuative, or 連用形{れんようけい} form of the verb. In fact it turns out that I keep asking questions about this form from different angles, not realizing it's the same thing each time!

Now when I look up Wikipedia to learn more about this form and how it is being used in コーヒー割り I actually find that -i forms are usually used as prefixes.

So this must be an "unusual" case since it appears to be being used like a suffix. What is this particular use of the -i form? How can I understand and use constructions of this type generally?

Answer

-i form of a verb, among other things, can be used to form nouns that are derived from this verb. For example

to discount (v) -> discount (n): 割り{わり}引く{びく} -> 割引{わりびき}

to rest/to have take a day off (v) -> rest/holiday (n): 休{やす}む -> 休{やす}み

to apply (v) -> application (n): 申{もう}し込{こ}む -> 申{もう}し込{こ}み

So I believe this is not the case of a verb form being a suffix but rather of a noun formed from a verb.

This is also similar in case of words from your previous question. For example, my dictionary shows the following example for 水割り, which suggests it being a noun:

水割りをもう一杯ください。