na-adjective
making light of; disregarding; treating as unimportant
Used adverbially, typically as あだおろそかに followed by a negative verb, to express that something should not be taken lightly or neglected. The word is a combination of 徒 (あだ, vain/futile) and 疎か (おろそか, neglectful).
See also: 徒や疎か (あだやおろそか)
彼の忠告をあだおろそかにしてはいけない。
You must not make light of his advice.
健康をあだおろそかにすると、後で後悔する。
If you disregard your health, you'll regret it later.
Compound of 徒 (あだ, 'vain, futile') and 疎か (おろそか, 'neglectful'). The exact historical derivation is uncertain, but the combination intensifies the sense of careless disregard.