Jamieson, Fausset and Brown on Psalms 35:15-16

15-16. On the contrary, they rejoiced in his affliction. Halting, or, "lameness," as in Psa 38:17 for any distress.

abjects--either as cripples (compare 2Sa 4:4), contemptible; or, degraded persons, such as had been beaten (compare Job 30:1-8).

I knew it not--either the persons, or, reasons of such conduct.

tear me, and ceased not--literally, "were not silent"--showing that the tearing meant slandering.