The word 'sward' came up in a conversation I was having earlier. It's either a poetic or obsolete word these days, I think,and means grass or a grassy surface. The word got me thinking about the similar word 'sword', and I wondered why it was that the w of sword is not pronounced. As far as I am aware, it's the only word beginning sw where the w is silent - swap, swarm, swing, sweater etc all pronounce the w, as does swore and sworn which differ by only one letter from sword.
Both sword and sward are very old words; both were around well before the Battle of Hastings. Sword was written sweord and sward was written sweard. Perhaps the w of sword was pronounced at the time and the different vowel sounds made it clear which word you meant (if the context, for some reason, didn't). These days sword and sward rhyme; the w sound is the only difference in pronunciation.