I just found myself using "umquhile" in another thread. I don't recall ever needing it before; intuitively "erstwhile" didn't quite convey the same shade of meaning but I'm not sure why not. (Maybe "umquhile" refers to former existence, "erstwhile" only to former attributes - your erstwhile bank manager might be in jail for fraud, your umquhile bank manager has to be dead).

The life experience that did most for my vocabulary was getting hepatitis A when I was a kid and spending three weeks in bed vomiting every half hour or so. Reading anything that required extended attention was out; following links through the Chambers Twentieth Century Dictionary was fascinating. But I have still never found a way to drop "omneity" into a conversation.