If we try to pronounce the word in English instead of in Klingon, we might use these pronunciations:
gh, pronounced as in tough
o, pronounced as in women
ti, pronounced as in nation.
Resulting in something that sounds exactly like "fish". It's a joke about how crazy English spelling is and it's been around since 1855. So Dr. Okrand incorporated it into the Klingon language where pronunciation is much more standard and so you wouldn't immediately recognize it on hearing it.