ad hoc
Latin for for this. Two words, and no part of a pig is involved.
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Wrong: We’ll do it on an add hock basis.
Right: We’ll do it on an ad hoc basis.
Latin. Ad hoc: for this, and for nothing else.
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Wrong: They set up an adhoc committee.
Right: They set up an ad hoc committee.
Two words. Same ad as in ad hominem.
Where it comes from
Latin, for this. An ad hoc committee is convened for one job and then dissolved; an ad hoc fix is one that solves the case in front of you and generalizes to nothing.
That is the useful part of the phrase, and it is the part people drop when they use it to mean “improvised.” Improvised is about how carefully it was made. Ad hoc is about how narrowly it applies.
Why nothing catches it
Hock is a real English word. It is a joint on an animal’s hind leg, and it is where your watch goes when you pawn it. Spellcheck therefore has no opinion, and add hock sails into the meeting notes unchallenged.
In fairness
Hyphenating it as a modifier—an ad-hoc process—is a defensible style call, and you will see it in print from people who know exactly what they are doing.
Add hock is not a style call.
We’ll sort the rest out on an add hock basis.