The word ‘Waheguru’ is combination of first character of the names by which god was called in different four yugs (ages), i.e
W – Wishnu (Vishnu)
H – Hari (Krishan)
G – Gobind
R – Ram
As in gurbani each and every word has a meaning. Even if a word is spelled differently in Gurbani, has a different meaning. Gurbani is complete. We can not say; Even without this word the meaning of stanza (or line) will remain same and complete. Same way, except W, H, G, R the sounds aa( for WA), ae (for HE), u (for GU) and uu (for RU) must also have some meaning. The word ‘Waheguru’ has a deep meaning.
My guru, Sant Gurnam Singh Ji taught me, so I believe it.
WA = Wah = Wow = Astonishing
HE = Everywhere/Anywhere
GU = Dark
RU = Light
He taught me Waheguru is he who can illumine (anything) anywhere by vanishing dark in an astonishing manner.