WordNet (r) 3.0 (2006):

high sign
    n 1: a silent signal of warning or recognition; "she started to
         speak but he gave her the high sign"

Moby Thesaurus II by Grady Ward, 1.0:

97 Moby Thesaurus words for "high sign":
   Roman candle, SOS, aid to navigation, alarm, amber light, balefire,
   beacon, beacon fire, bell, bell buoy, blinker, blue peter, buoy,
   caution light, danger sign, early symptom, falling barometer,
   flare, fog bell, fog signal, fog whistle, foghorn,
   gathering clouds, glance, go light, gong buoy, green light,
   heliograph, international alphabet flag,
   international numeral pennant, kick, leer, marker beacon, nod,
   nudge, office, omen, parachute flare, pilot flag, poke,
   police whistle, precursor, preliminary sign, premonitory sign,
   premonitory symptom, prodroma, prodrome, quarantine flag,
   radio beacon, red flag, red light, rocket, sailing aid, semaphore,
   semaphore flag, semaphore telegraph, sign, signal, signal beacon,
   signal bell, signal fire, signal flag, signal gong, signal gun,
   signal lamp, signal light, signal mast, signal post, signal rocket,
   signal shot, signal siren, signal tower, skull and crossbones,
   spar buoy, stop light, storm petrel, stormy petrel, symptom,
   the nod, the wink, thundercloud, thunderhead, tip, tip-off, touch,
   traffic light, traffic signal, warning, warning sign,
   warning signal, watch fire, white flag, wigwag, wigwag flag, wink,
   yellow flag, yellow jack