He also called him this:
You can interpret it two ways:
A. Shanks was strong enough to beat Loki but still didn't want to fight the WG, hence he was calling him a coward.
B. Loki was tricked by Shanks in someway and hates him because he didn't vanquish him fairly.
I think B is at least a possibility, partly because of Oda's emphasis on Loki's unnatural power and the deception involved in his past, partly because of the parallel to Fenrir being chained.
Ofc A is also possible due to Oda being a Shanksphile, but I don't really want it to happen.