serious-mindedness

NOUN
18 LETTERS · STARTS WITH S

serious-mindedness is a valid 18-letter word worth 21 points in Scrabble and 25 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS
S1E1R1I1O1U1S1-0M3I1N1D2E1D2N1E1S1S1
1 + 1 + 1 + 1 + 1 + 1 + 1 + 0 + 3 + 1 + 1 + 2 + 1 + 2 + 1 + 1 + 1 + 1 =21 PTS
DOUBLE WORD → 42TRIPLE WORD → 63
MEANING

What does serious-mindedness mean?