“Hotter than a jalapeño’s armpit” means very hot. A jalapeño pepper can be very hot to eat, but it doesn’t have armpits. The expression probably borrows from “hotter than the devil’s armpit” or “Hotter than Satan’s armpit.”
 
“it’s hotter than a jalapeño’s armpit in my house right now smh” was posted on Twitter by Quey Freeman on June 30, 2010. “naw naw it’s hotter than a jalapeño’s armpit after a ten mile run in the Mohave desert…” was posted on Twitter by Oro Cinco on August 21, 2011. “It is hotter than a jalapeño’s armpit” was printed in the article “Heaton: 10 Southern Sayings for Sweltering Summer Heat” in Hot Toddy on June 4, 2015. It is not known who originated the saying.
   
   
Wikipedia: Jalapeño
The jalapeño (UK: /ˌhæləˈpeɪnjoʊ/ HAL-ə-PAY-nyoh, US: /ˌhɑːl-/ HAHL-, Spanish: [xalaˈpeɲo]) is a medium-sized chile pepper pod type cultivar of the species Capsicum annuum. A mature jalapeño chile is 5–10 cm (2–4 in) long and hangs down with a round, firm, smooth flesh of 25–38 mm (1–1+1⁄2 in) wide. It can have a range of pungency, with Scoville heat units of 4,000 to 8,500. Commonly picked and consumed while still green, it is occasionally allowed to fully ripen and turn red, orange, or yellow. It is wider and generally milder than the similar Serrano pepper.
     
Twitter
Quey Freeman
@Slysquadq
it’s hotter than a jalapeño’s armpit in my house right now smh
9:38 PM · Jun 30, 2010·Twitter for iPhone
   
Twitter
Oro Cinco
@5onthemove
@itsLyNC naw naw it’s hotter than a jalapeño’s armpit after a ten mile run in the Mohave desert…
9:57 PM · Aug 21, 2011·Twitter for iPhone
   
Twitter
Douglas Greene
@nineafterten
It’s my last week in NC so of course the weather is hotter than a jalapeno’s armpit. #Only89DaysTillFall
11:12 AM · Jun 25, 2012·tGadget
 
Hot Toddy
Heaton: 10 Southern Sayings for Sweltering Summer Heat
JUNE 4, 2015
(...)
7. It is hotter than a jalapeño’s armpit.
   
Twitter
Sour Bags & Totes
@SourBagsTotes
It is hotter than a jalapeno’s armpit out there! I shall remain in the comfort of my air conditioned space and make lots of bags. #etsymtl
7:01 AM · Jul 29, 2015·Twitter Web Client
     
Twitter
docrock
@docrock
Hotter than a Jalapeño’s armpits outside #dreamfearlessly http://mapmyfitness.com/workout/1428051363
6:57 PM · Apr 14, 2016·erased338647
 
Twitter
strawberry cream pie
@cariebeara
Dude it is hotter than a jalapeño’s armpit in this store
7:19 PM · Apr 19, 2016·Twitter for Android
     
Twitter
Daniel W-Watts ♿
@Mr_NME
Replying to @Cardifolderol
It is hotter than a jalapeño’s armpit. #SwelteringSummerQuotes
10:47 AM · Jul 19, 2016·Twitter for Android
 
TV Fanatic
Man is it hotter than a jalapeño’s armpit out there.
—Cabe  
Episode:  Scorpion Season 3 Episode 12: “Ice Ca-Cabes”
Show:  Scorpion
Related Quotes:  Scorpion Season 3 Episode 12 Quotes, Scorpion Quotes
Added by:  Allison Nichols
Added:  January 02, 2017
   
Twitter
wxornotBG
@wxornotBG
Today 7/20: Hotter than a jalapeño’s armpit.
🌡Now 73°📈95°
💨SW @ 5-15 mph
7:40 AM · Jul 20, 2017·Twitter for iPhone
 
Newspapers.com
21 June 2018, The Herald-Sun (Durham, NC), “How hot is it? ‘Hotter than Satan’s house cat;’ 86 ways to say it’s hot” by Josh Shaffer, pg. 4A, col. 2:
6. Hotter than a jalapeño’s armpit.
 
Twitter
A Southern Nobody 🃏
@ExLibrisNemo
It’s hotter than a jalapeño’s armpit outside.
2:09 PM · Jun 12, 2021·Twitter for Android
     
Twitter
Major Tom (Sara)
@SassyPants0210
Replying to @LiamLong85
Here in NC it’s hotter than a jalapeño’s armpit. 😂🥵
12:14 PM · Jun 23, 2022·Twitter for iPhone
 
Twitter
Balki Bartokomous
@wargote4
Looking like tomorrow will also be hotter than Satan’s toenails, hotter than a blister bug in a pepper patch, hotter than a jalapeño’s armpit, hotter than a fire hydrant chasing a dog, hotter than a steel playground at recess, hotter than a honeymoon hotel,
12:37 AM · Jun 24, 2022·Twitter for iPhone