Very few words in English contain three consecutive letters of alphabet, with the exception of one of these combinations which has at least 19 words - not including the different forms of those words.  Listed below are seven of eight such combinations, with the number of known answers in parentheses. Can you come up with the words of these eight, and  identify the missing combination with its 19 examples?

DEF (1)
FGH (1)
GHI (1)
HIJ (1)
LMN (1)
NOP (1)
RST (2)

Answers:

DEF - DEFT
FGH - AFGHAN
GHI - LAUGHING
HIJ - HIJACK
LMN - CALMNESS
NOP - INOPERATIVE
RST - FIRST - THIRST

Missing combination: STU:

ASTUTE
STUB
STUBBORN
STUD
STUDENT
STUDIO
STUDY 
STUFF
STULTIFY
STUMBLE
STUMP
STUN
STUNT
STUPID
STUPENDOUS
STUPOR
STURDY
STURGEON
STUTTER