Rob Kardashian says his relationship with Blac Chyna started when he was going through the worst part of his life, and says he didn't even love her when they got engaged ... all during bombshell testimony.

Rob took the stand Wednesday in the case between Chyna and his family. He said things in his relationship with Chyna happened fast ... and thought she was right for him because she accepted him when he was going through his "lowest point."

He testified things went south quickly, and when he asked her to marry him, he didn't even love her ... "It wasn't real love. I had a baby with this girl."

Rob testified Chyna beat him at least 5 times over the course of their relationship. He continued, "I had a gun held to my head several times by this woman. That's not love."

Rob also detailed the night when Chyna allegedly strangled him with a phone cord, saying she'd been doing cocaine and drinking all night. He says the incident caused him to hire full time security out of fear for his life and for his daughter, Dream.

As we've reported, all of Rob's sisters, except Kendall and Kourtney, have taken the stand over the last couple of weeks. During Khloe's testimony, emails she had allegedly sent E! execs were read in which she expressed concerns Rob and Chyna's turbulent relationship could hurt the family brand ... she also suggested she might leave 'KUWTK' if the volatile behavior continued.

In other testimony, Kris Jenner and Corey Gamble say they believed Chyna had posed a very real threat to Rob's life ... Gamble says he intervened one night during a nasty fight between the couple -- where Chyna allegedly threatened to kill Rob.

Chyna's suing the family for $100 million ... alleging they torpedoed her "Rob & Chyna" show and in turn, caused her to lose out on a ton of money.