1 - having a harmful effect, eg to health
2 - the panicky paranoid state of mind which inspires one to go through old emails, texts, etc, deleting anything which may be vaguely incriminating
I found my girlfriend going through my phone, but it's ok cos I got a bit deleterious last week and wiped those drunk text messages to my ex from my outbox
Causing harm or damage.
Divorce is assumed to have deleterious effects on children.