Mnemonic opens with mn-, a cluster English cannot pronounce and does not try to — the m is simply silent. That is a Greek signature, and it sits with ps- in psalm, pn- in pneumonia, pt- in pterodactyl and x- in xylophone. Greek allowed these clusters at the start of a word, Latin transliterated them faithfully, and English inherited spellings it can only manage by dropping the first letter.
The word comes from Greek mnēmonikos, "of or pertaining to memory," from mnēmē, "memory," from an Indo-European root meaning "to think" — the same root, at a great distance, behind mind, mental and mania. English formed it in 1753 as a back-formation from mnemonics, and the noun sense, a mnemonic device, is later still, from 1858. In Greek mythology the mother of the Muses was Mnemosyne, Memory.
mnemonic (adj.) 1753, "aiding the memory, intended to assist the memory;" 1825, "pertaining to the memory," a back-formation from mnemonics , or from a Latinized form of Greek mnēmonikos "of or pertaining to memory," from mnēmōn (genitive mnēmonos ) "remembering, mindful," from mnēmē "memory, a remembrance, record, an epitaph; memory as a mental faculty," from base of mnasthai "remember," from PIE root *men- (1) "to think." The noun meaning "mnemonic device" is from 1858. Related: Mnemonical (1660s). also from 1753
mnemonic came in through Greek, and its spelling follows that family's conventions.
How that family writes these sounds
| a bare /n/, /t/, /s/, /z/ at the start | put the silent letter back on the front: pn-, pt-, ps-, mn-, x-pneumonia, pterodactyl, psalm, mnemonic, xylophone |
What gives it away on the page
| ps-, pn-, pt-, mn-, x- | Initial clusters English can't actually pronounce, so it silences the first letter.psalm, pneumonia, pterodactyl, mnemonic, xylophone |