Definition of boo:
(interjection) :
a
loud exclamation intended to
scare someone, especially a child. Usually used when one has been
hidden from the
victim and then suddenly
appeared
unexpectedly; a word used
ironically in a situation where one might have scared someone, but said someone was not scared. Not said as loudly as in definition 1
(n) :
a derisive shout (as interjection definition 4 above); significant other. (Ex. John is my boo.)
(v) :
to shout extended boos derisively; to derisively shout extended boos at
"when he took the podium, the crowd booed."