Angus Cloud's Dad Died From 'Brief' Cancer Battle Months Before Actor's Death

It's becoming more clear why Angus Cloud's family says he was struggling so much with the loss of his dad ... because his father's life was cut short after a very quick battle with cancer.

65-year-old Conor Hickey, Angus' dad, passed away on May 18 -- a social media post reshared by the "Euphoria" star's mom, Lisa, said he died "after a brief fight with cancer."

Lisa shared her own thoughts in June after his memorial service -- saying, "His cancer moved quickly which was devastating to all who loved him, but there is comfort in the fact that his pain wasn't prolonged."

TMZ broke the story, Angus died at his family home Monday, with Oakland PD and Fire Dept. responding to a 911 call from Lisa about a "possible overdose," claiming her son didn't have a pulse.

Angus' family told us he's been struggling with the loss of Conor, whom they say was his "best friend," adding Conor was buried just 1 week before. Angus posted about his dad a couple of weeks ago, letting him know he's missed.

A source close to the family told us Angus had been struggling with severe suicidal thoughts after getting back from the burial in Ireland ... and he was staying with his family to try and work through the grief.

Angus' cause of death has yet to be determined.