From the IPA pronunciation guide of Oliver Heaviside's wikipedia page (mouse over the bit in parenthesis right after his name at the top of the page), "heavy side", with a slight emphasis on the first syllable of "heavy".

https://en.wikipedia.org/wiki/Oliver_Heaviside

At university in the UK it was always pronounced heavy-side.

My first interpretation was heveh-side or hevi-side.

I have no idea what the correct pronunciation is. But between you and me, I think we have some viable candidates.

Heavy side - it’s a pun