Bob Bolton; this cuts both ways. "Waelisc" (or something like it) = Anglo-Saxon "foreigner"

However, the Welsh (Cymraeg) word, "Saeson", which obviously means "Saxon", has come down the ages as "Sais" which means "English" (person) or "bl***y foreigner".

Other than place-names, or parts of place-names (such as all those rivers named "Avon", after the Welsh "afan", meaning, "river") I can think of very few Welsh words which found their way into English. Those which did aren't usually very complimentary. "Glib" (= shallow, ready, insincere), is one such, from the Welsh "gwlyb", which means "wet", or "slippery". I'll try and think of a few more.