I swear vs I'm not fucking lying

When to use each in English, with meaning, register, and examples.

I swear

Informal Top 2,000 (common)

I'm not fucking lying

Slang Top 5,000 (fairly common)

Most formal: I swear Most common: I swear

	I swear	I'm not fucking lying
Pronunciation	🇬🇧 //aɪ swɛər// 🇺🇸 //aɪ swɛr//	🇬🇧 //aɪm nɒt ˈfʌkɪŋ ˈlaɪɪŋ// 🇺🇸 //aɪm nɑt ˈfʌkɪŋ ˈlaɪɪŋ//
Meaning	I promise something is true	I am not telling an untruth.
Example	I swear I'm telling the truth!	I swear, I'm not fucking lying when I say I can run a marathon.
Register	Informal	Slang
How common	Top 2,000 (common)	Top 5,000 (fairly common)
Collocations	swear to tell the truth, swear on my life, swear under oath	not fucking around, not fucking joking, fucking serious
Common mistakes	Using 'swear' without 'I' (like 'swear it's true') - it sounds unnatural., Confusing with 'promise' – 'swear' is stronger and often more emotional., Forgetting to use it only in informal settings.	Using 'lying' when they mean 'laying', Confusing the tone; this phrase can be too aggressive in formal contexts, Neglecting the informal nature of the phrase in professional settings
Usage notes	Use in casual conversation to emphasize the truthfulness of a statement. It can express strong feelings. Not appropriate for formal contexts.	This phrase is informal and can be strong or rude due to the profanity. It is used to emphasize that someone is being truthful.

See it in real clips

let me say that I swear

How I Met Your Mother › S1 E1

I didn't, I swear.

let me say that I swear

How I Met Your Mother › S1 E1

I didn't, I swear.

I swear

Peaky Blinders › S1 E1

I'm not fucking lying, all right?

I'm not fucking lying

Frequently asked questions: I swear vs I'm not fucking lying

What's the difference between I swear and I'm not fucking lying?

I swear: I promise something is true I'm not fucking lying: I am not telling an untruth.

Which is more formal: I swear and I'm not fucking lying?

I swear is the most formal of these.

Which is more common: I swear and I'm not fucking lying?

I swear is the most common in everyday English.

Can you show an example of each?

I swear: I swear I'm telling the truth! I'm not fucking lying: I swear, I'm not fucking lying when I say I can run a marathon.

Can I use I swear and I'm not fucking lying interchangeably?

Not always. I swear and I'm not fucking lying are related and overlap in some contexts, but they differ in register, how common they are, and usage, so swapping one for another can change the meaning or tone. Check the differences above before substituting.