Just as "underwhelmed" has been derived from "overwhelmed," "whelmed" in the new sense of "no special effect; eliciting a feeling of indifference" has entered the vocabulary recently (in the last 20 years, in my experience) as a backformation of sorts from "overwhelmed." My American Heritage Dictionary doesn't yet include the new meaning of "whelmed" in its definitions, but I'd bet that'll change soon enough.

Aside: while typing this, I managed to write "volcabulary" ... which I now take the opportunity to define as "terms related to the field of volcanology."