Someone has recently messaged me the phrase “if a dog bites once”, I do not understand what this phrase could mean and my friends do not understands it either. I’m getting a threatening feeling off it haha, thanks for answers!!
If a dog bites once, it will likely bite again with the second attack often more vicious that the first. sciencedaily
Typically a metaphor: if someone can do something bad once he or she can do it again. Be forewarned.
It is probably an extended reference to the saying...
"Once bitten, twice shy"
"...said when you are frightened to do something again because you had an unpleasant experience doing it the first time."
In other words:
Your original quote shows few hits.