What is a "loose" opening?

I'm starting to realize I don't know what "loose" means, in context of chess openings/positions. Korn uses this term repeatedly in his book MCO. I've been assuming it means something like pieces being placed according to tactics instead of according to long-term strategy, or of the position because of being awkward and prone to strategic attacks, or maybe of exposing the king too much, but I really don't know.

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(p. 23)
Another satisfactory system is the Open Defence (cols. 101-35)
consistently practised by Tarrasch and Euwe but currently out of
vogue. Black's game is slightly loose and White's "jump" is more
apparent when the lines are opened after 4 B-R4, N-B3; 5 O-O,
NxP. Nevertheless Black's resources are resilient and he can equalize
in the main lines. Deviations for White before move 5 are covered
in cols. 136-40. Noteworthy is the straightforward Exchange Variation
(col. 140), where White forces an end-game in which his superior
pawn-structure is counter-balanced by Black's two Bishops.

(p. 483)
The accelerated fianchetto with 3 P-KN3 (cols. 6-7), also known
as Carl's Bremen System, is less committal since White keeps the
diagonal open before playing N-KB3. Keres' 3...P-B3 is a
dynamic and effective way of securing equality (col. 7).
The Dutch pattern (col. 8) is a little "loose" but also secures even
chances. Against 2...NQB3 (cols. 9-10) White gets the edge by
playing a reversed Closed System of the Sicilian where he breaks with
P-K3 and P-Q4.

Evans, Larry, and Walter Korn. 1965. Modern Chess Openings, 10th Edition. New York: Pitman Publishing Corporation.

loose pieces I would guess?

I've heard the term used for king safety, but this is the first to describe an entire position.  I guess it means subject to attack.  Awkwardness if not downright danger might ensue, but you're not necessarily in trouble.  

Not solid.

Thanks. Your simple answer, though I first thought you might be joking, is actually probably the best: Korn probably needed an antonym for "solid," and chose that term since it's the opposite of "secured," otherwise known as "secure," which means "safe." So "loose" is probably synonymous with "unsolid" or "unsound" or "insecure" or "unsafe."

Madonna

What would be a "loose" pawn structure, per your understanding? Have you seen that term used before in conjunction with pawn structures, especially in some book?

The 4 pawns attack variation of the Alekhine is an example.

Overextended is similar.