The word advice self-contains the meaning that it is an activity done in good intentions, as compared to a mere suggestion.
guidance or recommendations offered with regard to prudent action
synonyms: guidance, advising, counselling, counsel, help, direction, instruction, information, enlightenment; recommendations, guidelines
Since the idea is to communicate that the 'Advice' was not well-meant, you may refrain from actually calling it an advice.
'Offhand' is an adjective that means with little or no preparation or forethought. The adjective offhand describes a remark or comment that is made without previous thought or preparation.
Another common word to consider is Casual
made or done without much thought or premeditation
"a casual remark"
synonyms: offhand, random, impromptu, spontaneous, unpremeditated, unthinking, unstudied, unconsidered, parenthetical, passing
Another word 'Nonchalant' is used to depict the attitude the person who is speaking has adopted towards a situation.
Nonchalant - Casually unconcerned or indifferent.