A mnemonic is a memory device for reducing something diverse and complicated to an easily -remembered pattern. For example, for the order of planets in the solar system, I learned as a boy the sentence:
Men very easily make jugs, serve useful necessary purposes; to give Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto.
Is there an adverb mnemonically, as in I am able to recite all the kings and queens of England, mnemonically.
Yes, of course there is. There HAS to be: you can always add ‑ally to an ‑ic adjective to derive an adverb of manner from it. The key here is realizing that mnemonic can serve as an adjective as well as it can serve as a noun.
No dictionary lists all possible words produced by derivational morphology. That doesn’t mean it isn’t a word. Obviously, it’s a word. In this instance, the OED even happens to have a citation for it:
1867 Q. Rev. Oct. 427 ― Each one of these mysterious letters was taken, mnemonically, as the initial of some technical word that indicated one of these four methods.
Predictably, mnemonically simply means in a mnemonic manner, just like all such derived adverbs do, mutatis mutandis.
