eludere "elude, frustrate" + -ive

source: Lupin Central

difficult to find, catch, or achieve:

〈人･動物などが〉うまく逃れる; 捕らえにくい, 見つけにくい

〈結果などが〉手に入れにくい, 達成しがたい

e.g. success will become ever more elusive.

difficult to remember or recall:

理解しにくい, 捕らえどころのない; 思い出せない

e.g. the elusive thought he had had moments before.

elusively |əˈlo͞osivlē| adverb

elusiveness |əˈlo͞osivnəs| noun

early 18th century: from Latin elus- ‘eluded’ (from the verb eludere) + -ive.