Entry from OED Online

[Echoic; cf. BANG v., BUMP v.¹, CONK v.¹, etc.] 

    1. colloq.    a. trans. To hit (a solid surface or bony part of the body) resoundingly with or against something hard and unyielding. (See also quot. 1931.)
 
  1931 BROPHY & PARTRIDGE Songs & Slang 1914-18 (ed. 3) 286 To bonk, to shell; generally in the passive. 1937 N. HUNTER Professor Branestawm's Treasure Hunt i. 13 The carrier men..bonked and rattled and squerked the package through the almost too small doorway and set it down with a thump. 1938 PARTRIDGE Dict. Slang (ed. 2) Add. 981/2 Bonk,..to hit (v.t.) resoundingly: mostly Public Schools': from 1919. 1963 Observer 10 Nov. 1/1 Each time a golfer wearing it raises his head while making a shot the pendulum bonks him in the face. 1975 Verbatim Sept. 1/2 Similar is bonk (bang + conk) in ‘A door nearly bonked her in the face’. 1984 N.Y. Times 26 Feb. 55/1 This snake came out. My grandfather pulled this wrench out of the plower and he bonked it on the head.
 

    b. intr. To bang or bump; to make an abrupt thudding noise (see BONK n. 1).
 
  1960 K. AMIS Take Girl like You iv. 49 The heavy door creaked and bonked shut. 1967 Time 19 May 109/1 Skelton got a concussion bonking into a ‘break-away’ door. 1986 Washington Post 25 May H2/5 The dreaded saber-toothed tiger..came prowling by, bonking into things.
 

    2. trans. and intr. To have sexual intercourse (with). slang.
 
  1975 Foul Mar. 9/4 (caption) Rita is currently being bonked by the entire Aston Villains defence! 1984 MCCONVILLE & SHEARLAW Slanguage of Sex 34/2 ‘They're not even bonking any more.’.. Entirely cross-sexual, with women being just as likely to say they bonk as are men. 1985 Venue 26 Apr. 29 He goes home early, only to find his wife merrily bonking with another bloke. 1986 Look Now Oct. 88/2 It was rather a surprise..to learn that he had booked us on a singles holiday with the express intention of bonking his way back to happiness. 1986 Daily Tel. 29 Oct. 14/8 Fiona..has become so frustrated that she has been bonking the chairman of the neighbouring constituency's Conservative association. 1987 Sun 21 Feb. 20/6 Fans who were at the concert..were still convinced that Carol had ‘bonked’ the virgin — and one of the band. Ibid. 20/7 It certainly looked like she and the boy were bonking.
 

    Hence bonking vbl. n., (a) = BONK n. 1; (b) slang, sexual intercourse.
 
  1958 New Scientist 20 Nov. 1324/3 A simple, mass-produced car..must be expected to give up the ghost to a procession of rattlings and bonkings soon after it has travelled 50,000 miles. 1985 Venue 26 Apr. 30 Some midnight assignations, a bit of bonking and a good deal of philosophising are obviously par for the course. 1986 Guardian 11 Nov. 26/3 A witty, blurred focus shot of a beaten-up 2CV in a romantic landscape, rocking with the steamy bonking of its occupants.