ad·ven·ti·tious [ad-vuh n-tish–uhs] Pronunciation Key – Show IPA Pronunciation–adjective
1. associated with something by chance rather than as an integral part; extrinsic.
2. Botany, Zoology. appearing in an abnormal or unusual position or place, as a root.
Dictionary.com Unabridged (v 1.1)
Based on the Random House Unabridged Dictionary, © Random House, Inc. 2006.
Short Definition: abnormal
His arrogant behavior today was adventitous; he's normally quite humble and polite.
We found a small gemstone in the mine, but it's presence must be adventitious as this region is not known for such gems.
Hat vent tissues > ape snore mole – The Cat in the Hat's hat is so tall it has vents on the side to help with circulation. The cat carries around a gigantic box of tissues to wipe the vents clean. He can't reach the vents while he's wearing the hat, however, so he's followed by a trained purple ape who sounds like he's constantly snoring. However, when you look closely, you notice that the snoring is coming from a giant hairy mole on the side of the ape's neck (that's an adventitious mole).
With this word, the short definition is so abstract that I encoded it with a mnemonic as well.