This counter, applicable only to InfiniBand switches, tracks outbound packets discarded because they exceeded the head-of-queue lifetime limit while blocked at the front of the transmit queue. A rising count signals head-of-line blocking severe enough that packets are timing out, usually caused by a slow or unresponsive downstream link holding up the queue. An admin seeing this counter climb would look for a congested or misbehaving downstream port that's backing up traffic on this switch.