Why is there a 'p' in “assumption” but not in “assume”?

Question

I know a little bit about the suffix -tion. It is usually added to verbs.

Examples:

-domination (from dominate),
-admiration (admire),
-deviation (deviate),
-ejection (eject).

"Exemption (exempt)" has a 'p' in the '-tion' form, because the verb form also has a 'p'. "Preemption (preempt)" also has a 'p' in both forms.

But "assumption" from "assume", the verb has no 'p' (like it's not "assumpt") and the noun derived from it has a 'p'.

It threw me off while I was thinking of the nouns and verb forms that are related. Why is there a 'p' in "assumption" but not in "assume"?

Answer 1

Assumption is directly derived from Latin assumptionem which does have a P, so it also has a P. Assume on the other hand is derived from Latin assumere, which didn't have a P.

Other similar examples include presume/presumption and consume/consumption

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Edit*

Articulatory reasons for the P

The epenthetic P is probably inserted for articulatory reasons.

Epenthesis happens for a variety of reasons such as:

• ease of articulation
• to prevent adjacent vowels in a hiatus (e.g. idea of being pronounced idea[r]of in most non-rhotic accents)
• to simplify consonant clusters (some people—mostly non-native speakers who don't have complex clusters in their native language—pronounce words like screen, scratch, school with a preceding vowel to break the consonant cluster).

Epenthetic stops between nasals and fricatives

There are many situations where epenthetic consonants are inserted. Most of the time when there's a voiceless fricative (/s ʃ θ/ etc) after a nasal (/m n ŋ/), we tend to insert an epenthetic stop (/p t k/ etc) between both the fricative and a nasal. The reason is because the air comes out through the nose while articulating a nasal and as the nasal changes to a fricative—an oral consonant—the airflow must be switched from nasal to oral and should be stopped before articulating an oral consonant, so there is a brief period in which both the nasal and oral airflow are stopped, this is a brief oral stop, homorganic (same place of articulation) with the nasal.

The same thing happened to assumption in Latin before it entered English (as Colin Fine explained). There's a voiceless fricative /ʃ/ after a nasal /m/, so when changing from nasal to oral, Latin speakers put an epenthetic stop /p/ (homorganic with the nasal) between the nasal and the fricative.

The stop is more likely if the fricative is voiceless, when the articulatory system has an additional voicing change to handle. It’s less likely if the fricative is at the beginning of a stressed syllable as in in'sist (no epenthetic stop).

There are many examples of epenthetic stops in present day English:

• youngster is usually pronounced young[k]ster, (with an epenthetic k between the nasal [ŋ] and the oral fricative [s])
• warmth and hamster are often pronounced warm[p]th and ham[p]ster, respectively (with an epenthetic p)
• thunder used to be þunor, the d is epenthetic.
• In some English accents, wants and once are pronounced identically (i.e. 'once' with an epenthetic t).

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*^{: Edited after Colin Fine's answer because I was in the middle of explaining when they posted their answer.}

Answer 2

Void's answer give the immediate explanation: English does it because Latin does it. Edit - and now also explains the phonetic reason.

But there is a more general answer behind this. It's a phenomenon called epenthesis (not a very good article, but it gives the idea): where a sound comes to be inserted between two other sounds, just because it starts happening by accident, and sometimes becomes standard.

Latin speakers (before the time of Classical Latin) came to pronounce historical *sum-si and *sum-tom as sumpsi and sumptom (classical sumptum) simply because, in moving the mouth from /m/ to /s/ or /t/, if the timing went a bit awry, a /p/ would just "happen".

Almost the same thing happened in the English word thimble: in Old English it was þȳmel ("thymel").