“Season” sounds like “sees in.” A joke was posted on Twitter by Bob Kostic (and others) on July 29, 2014:
   
“My brother is dating a girl named Rosemary. I don’t know what he season her. #sickipedia”
   
The joke probably originated on the Sickipedia website. “My brother is dating a girl named Rosemary…I don’t know what he season her! I guess I’ll understand in thyme!” was posted on Twitter by Jokerswild on September 12, 2019.
   
     
Wikipedia: Rosemary
Salvia rosmarinus, commonly known as rosemary, is a woody, perennial herb with fragrant, evergreen, needle-like leaves and white, pink, purple, or blue flowers, native to the Mediterranean region. Until 2017, it was known by the scientific name Rosmarinus officinalis, now a synonym.

It is a member of the mint family Lamiaceae, which includes many other herbs. The name “rosemary” derives from Latin ros marinus (“dew of the sea”). The plant is also sometimes called anthos, from the ancient Greek word ἄνθος, meaning “flower”. Rosemary has a fibrous root system.
   
Twitter
Alan Bret Vaughan
@Welsh_Gingerbal
My brother is dating a girl named Rosemary.
I don’t know what he season her.
2:54 PM · Jul 29, 2014 from Cardiff, Wales·Twitter for Android
   
Twitter
The Souls That Said No
@Snikoggs
My brother is dating a girl named Rosemary. I don’t know what he season her.
3:25 PM · Jul 29, 2014·Twitter for BlackBerry®
         
Twitter
Bob Kostic
@causticbob
My brother is dating a girl named Rosemary.
I don’t know what he season her.
#sickipedia
5:00 PM · Jul 29, 2014·Twitter Web Client
 
Facebook
Tonys funny Jokes page
July 30, 2018 ·
My brother is dating a girl named Rosemary…
I don’t know what he season her… 
   
Reddit—Jokes
Posted by u/FleshCoffin July 31, 2018
my friend just started dating a girl called Rosemary
I don’t know what he season her
COMMENTS
Thomas_Plunkett
Only thyme will tell.
bazognoid
Sage advice.
 
Twitter
Dad Jokes
@Dadsaysjokes
My brother is dating a girl named Rosemary…
I don’t know what he season her…
4:25 PM · Sep 10, 2018·Twitter for iPhone
   
Facebook
David’s Corny Jokes
March 6, 2019 ·
My friend is dating a girl called Rosemary.
I don’t know what he season her.
   
Twitter
Jokerswild
@JokersWildUK
My brother is dating a girl named Rosemary…
I don’t know what he season her!
I guess I’ll understand in thyme!
2:53 AM · Sep 12, 2019·Twitter for Android
 
Twitter
🤣 The Dad Joke Man 😉
@DadJokeMan
My brother is dating a girl named Rosemary…
I don’t know what he season her…
1:55 PM · Oct 13, 2019·Twitter for iPhone