Princeton's WordNet

1. sucking, suck, suction(verb)

   the act of sucking

   Synonyms:
   sucking, suck, suction

2. suck(verb)

   draw into the mouth by creating a practical vacuum in the mouth

   Synonyms:
   soak up, go down on, sop up, wet-nurse, fellate, draw, give suck, absorb, suckle, take in, suck in, nurse, imbibe, lactate, blow, take up, suck up, breastfeed

3. suck(verb)

   draw something in by or as if by a vacuum

   Synonyms:
   soak up, go down on, sop up, wet-nurse, fellate, draw, give suck, absorb, suckle, take in, suck in, nurse, imbibe, lactate, blow, take up, suck up, breastfeed

4. suck, suck in(verb)

   attract by using an inexorable force, inducement, etc.

   Synonyms:
   soak up, go down on, sop up, wet-nurse, fellate, draw, give suck, draw in, absorb, suckle, take in, suck in, nurse, imbibe, lactate, blow, take up, suck up, breastfeed, suck

5. suck(verb)

   be inadequate or objectionable

   Synonyms:
   soak up, go down on, sop up, wet-nurse, fellate, draw, give suck, absorb, suckle, take in, suck in, nurse, imbibe, lactate, blow, take up, suck up, breastfeed

6. fellate, suck, blow, go down on(verb)

   provide sexual gratification through oral stimulation

   Synonyms:
   swash, gasconade, take in, botch, suck up, bungle, shove along, muck up, mishandle, bumble, botch up, brag, blow, squander, burn out, flub, suck, waste, give suck, fumble, shove off, fellate, soak up, bluster, ball up, bollocks, suckle, bobble, mess up, nurse, louse up, bollocks up, float, gas, bollix, imbibe, vaunt, screw up, muff, shoot a line, blow out, breastfeed, draw, foul up, tout, fluff, sop up, drift, absorb, go down on, be adrift, lactate, bollix up, wet-nurse, take up, spoil, boast, bodge, suck in

7. absorb, suck, imbibe, soak up, sop up, suck up, draw, take in, take up(verb)

   take in, also metaphorically

   Synonyms:
   plunge, receive, occupy, withdraw, run, tie, ingest, draw, assume, resume, have, borrow, lactate, overhear, eviscerate, absorb, cast, take over, consume, earn, suckle, pull, draw and quarter, adopt, reap, draw in, scoop, fellate, bootlick, drink, gather in, steep, gull, get out, pull back, disembowel, collect, start, lift out, fool, suck, sidle up, breastfeed, put one across, truckle, go down on, seize on, realize, sop up, fasten on, blow, strike, take out, latch on, take up, delineate, see, pull in, take, quarter, catch, befool, make, hook on, wet-nurse, pull out, get, describe, puff, suck in, thread, soak up, drag, cotton up, shine up, attract, scoop up, cozy up, force, cod, toady, sorb, engage, trace, dupe, put on, kowtow, engulf, view, kotow, draw off, immerse, engross, invite, take in, pass, play up, give suck, line, slang, watch, fawn, suck up, bring in, clear, string, guide, depict, put one over, scoop out, assimilate, realise, imbibe, nurse, gain

8. breastfeed, suckle, suck, nurse, wet-nurse, lactate, give suck(verb)

   give suck to

   Synonyms:
   soak up, entertain, go down on, sop up, wet-nurse, fellate, hold, draw, give suck, absorb, suckle, take in, suck in, harbour, nurse, imbibe, lactate, blow, take up, suck up, harbor, breastfeed, suck