While looking stuff up, I became curious about how many words include the unusual pairing of m and n, and I discovered more than I had expected. Many more than twenty exist, but I’ve listed only that number (along with their definitions), choosing to exclude several categories of words. (See below for details.) Words ultimately derive from Latin by way of an earlier form of English or French unless otherwise indicated.
1. alumnus: a former student of a particular school (from a Latin root word meaning “to nourish”)
2. amnesia: loss of memory, a gap in memory, or, informally, selective memory (from a Greek word meaning “forgetfulness”)
3. amnesty: pardon or freedom for a group of people (related to amnesia)
4. amnion: a membrane around an embryo or fetus (best known in the adjectival form amniotic) (from the Greek word for “lamb”)
5. autumn: the season also known as fall, or a late stage of life or existence
6. calumny: slander
7. chimney: the part of a building that includes one or more flues for discharging smoke, or a similar-looking rock formation
8. column: a long vertical building support, a similar-looking structure, a vertical arrangement of text, a statistical category, or a long row of marching people
9. condemn: convict, doom, or sentence, or declare something wrong (the root word is related to damn)
10. contemn: treat with contempt
11. damn: condemn, or send to hell, or used as an oath or an intensifier
12. gymnast: an athlete who competes in exercise routines on a floor mat or on specialized equipment (from Greek)
13. hymn: a religious song (ultimately from Greek)
14. insomnia: inability to sleep
15. limn: describe, delineate, draw, or paint
16. mnemonic: relating to memory, or intended to assist in memory (from Greek)
17. omnibus: a large passenger vehicle (the full word from which bus is derived), or an anthology
18. remnant: a part left over from a whole
19. solemn: sad and or serious, or sincere
20. somnolent: sleepy, or causing one to feel sleepy (related to insomnia)
I’ve excluded the following categories of words: variations and inflectional endings, the dozen other words beginning with the prefix omni-, the nine words that end with m followed by the suffix -ness (such as calmness), obsolete words, proper names like the Native American place name Tuolumne, and obscure, little-used terms like simnel (referring to a crisp bread or a fruitcake) that no one but a serious Scrabble competitor would know.
An interesting list – many thanks. However, I was very surprised that you regard Simnel as such an obscure word – here in the UK the tradition of making Simnel cakes is alive and well, though I suspect the symbolism of the 11 (or sometimes 12) marzipan balls on the top is less familiar. I’ve never come across Simnel attached to any form of ‘crisp bread’ – a Simnel cake is a light fruit cake, with a layer of marzipan in the middle and another on top, decorated with the aforementioned marzipan balls.
I’m sure that as the day goes on, people will add their words to your list of 20. For some reason, the first word I thought of was indemnity and the next one was mnemonic. I think pronunciation bears mention for people whose first language is not English. For example, the N at the end is not pronounced (as far as I know), unless followed by something else (e.g. damn, damnation), but at the beginning like in mnemonic, the M is not pronounced.
Mnemonestenes was the Greek goddess of the memory.