From Wiktionary and other similar sources like etymonline, the meanings of "advert" and "adverse" are:
advert: turn attention
adverse: Unfavorable; antagonistic in purpose or effect; hostile;
actively opposing one's interests or wishes; contrary to one's
welfare; acting against; working in an opposing direction.
"ad-" means "to", while "verse" and "vert" both mean "turn".
How shall I understand that they share same etymology but have unrelated meanings?
Best Answer
The reason they have different meanings is that although the stem vertere is constant both the prefix and suffixes alter the stem significantly. The prefix ad- in the two words is used in different senses and only means toward in adverse.
Thus in advert the prefix ad- is cognate with a- just having the effect of adding - leaving the meaning of the stem verb unchanged and in the present tense (to turn).
whereas in adverse the prefix ad- means toward (cognate with at) and the stem is formed from the past participle verttus changing to versus meaning turned - thus giving the (past participle) sense of turned toward or opposed.
This also explains why advert as discussed in the sense here is always a verb whereas adverse is always an adjective (i.e a noun modifier used as past participle).