とんとん

〈物もの〉をとんとん(と)ノックするto knock tap-tap on something
彼かれはドアをとんとんとノックした。
He knocked tap-tap on the door.
〈事こと〉がとんとん(と)進すすむfor things to proceed smoothly without a hitch
交こう渉しょうはとんとんと進すすんで、すぐに契けい約やくが決きまった。
The negotiations went smoothly, and the contract was settled right away.
収しゅう支しがとんとんだfor income and expenses to break even
今こん月げつは収しゅう支しがとんとんだった。
This month, income and expenses came out even.
〈人ひと〉の肩かたをとんとん(と)叩たたくto tap someone lightly on the shoulder
後うしろから肩かたをとんとんと叩たたかれた。
Someone tapped me lightly on the shoulder from behind.

Related words

順じゅん調ちょうN3sense 2
とんとん is a casual, mimetic way to say things are going smoothly, while 順調 is a more formal, neutral adjective for smooth progress.
イーブン sense 3
とんとん and イーブン both describe a tied or break-even state, but イーブン is a casual loanword often used for scores or simple comparisons while とんとん is common in financial contexts too.
コンコン sense 1
とんとん and コンコン both mimic knocking sounds, but コンコン has a sharper, more hollow ring often used for knocking on doors or coughing.
丁とう々とうN1
丁々 is a literary, somewhat old-fashioned mimetic word for a chopping or thumping sound, while とんとん is the everyday, more common word for a similar rhythmic knocking or tapping sound.
すらすらN3
とんとん describes things proceeding smoothly and on schedule (often with 拍子に), while すらすら specifically describes fluent, effortless speech or writing.

Opposites

難なん航こうN1sense 2
とんとん describes things proceeding smoothly, whereas 難航 describes progress running into serious difficulty or delay.

Don't confuse with

どんどんN4sense 1
とんとん describes a light, soft tapping sound or smooth progress, while どんどん describes a loud, continuous banging sound or rapid, vigorous progress.
コツコツN2
トントン describes a lighter, quicker tapping or knocking sound (and can also mean 'even/on par'), while コツコツ tends to describe a duller, more deliberate repeated tapping.

Written forms

とんとんUsage31%
Standard hiragana spelling for this onomatopoeic word.
トントンUsage69%
Katakana spelling is common in casual writing, manga, and when emphasizing the sound.

Etymology

Onomatopoeic word mimicking a light tapping sound. The smooth-progress and even/break-even senses are figurative extensions of the balanced, rhythmic tapping image.