What is missing from this sequence of words?

What is missing from this sequence?

SUM, SUN, SIX, -, SIP

Source: GCHQ Puzzle book

Answer

My answer is:

SNORCIMO

Explaination:

The word sequence given contains in order the 12th (Mu), 13th (Nu), 14th (Xi), and 16th (Pi) letters of the Greek alphabet reversed with an added "S". The missing Greek letter in the order is the 15th letter of the alphabet (Omicron). Following the same naming convention gives you SNORCIMO:

Mu = SUM

Nu = SUN
Xi = SIX
Omicron = SNORCIMO
Pi = SIP