Literally, this means “of its own kind; unique”. It was being used in a legal sense, so it means “when a special and unique interpretation of a case or authority is found to be necessary”. (Wikipedia).
sui = of itself
generis comes from genus = kind or type
“Unique” itself comes from Latin and means “the only one of its kind”.
Uni is a Latin prefix (letters added (or fixed) to the beginning of a word to change its meaning) that means “one”.
Other words that start with uni include;
unicellular – consisting of one cell
unicorn – one horn
unicycle – one wheel
uniform – one form, the same
unify – to make or become one
unilateral – from or to one side
union – two or more things joined into one
unison – literally “one sound”, unison means being together in harmony at the same time.
unite – to join together or combine into one.
unity – oneness, being one.
universe – the whole of creation, from the Latin universus which means “combined into one.”