Waboflex is a type of expansion joint which is particularly useful in bridge construction. It is made from moulded neoprene1, reinforced with steel plates. The top portion is protected from the traffic by a ribbed aluminium plate. Once installed, the joint will be water tight and self-draining, and support heavy traffic loads. Movements of around 300mm (about 1 foot) are supported.

Expansion joints are required because metal expands in the heat, and contracts in the cold. Therefore, bridges and other large structures employ joints such as the Waboflex to accommodate the changes of shape.

Perhaps because it's an odd-sounding word, it's cropped up in the chatterbox lately as an exclamation. See the e2 Chatterbox Archive.

1: Kidas informs me that it may be EPDM, a synthetic rubber.

A picture of a waboflex joint is here: http://www.spennteknikk.no/HjemmesideEng/produkter/prod_wflex.html
A technical description of the "Waboflex SR" joint is here: http://www.mbtfeb.co.uk/htmfiles/waboflexsr.htm

In case you're wondering:
Infinite Burn says wab, rhymes with flab... and the rest is o-flex, rhymes with go sex.