What do Japanese Mean by « Grinding Sesame Seeds » ?

Essential Ingredient for Japanese Cuisine : Sesame Seeds

In Japan, sesame seeds are often used in cooking. The seeds are sprinkled over dishes, and made in to a paste, or sesame oil. I love sesame oil and use it almost every day in my cooking.

Sesame seeds contain a lot of oil, so if you grind them well to make a paste, the oil will gradually come out and make the paste a little oily and sticky.

That's why the expression "grinding sesame seeds" got to be used to describe a person who hangs around someone to flatter them. So, the answer is (2) flatter!

There are many metaphorical expressions to mean "flatter" in English as well, such as 'to butter up', 'apple polish', to be a 'crawler' etc... 

How is it used?  ごまをする "Goma o suru"

This expression in Japanese is ごま を する (Goma o suru). ごま (goma)  is sesame seeds, を (o) is the particle indicating that the previous word ("goma" in this case) is the direct object, する (suru) is the verb "grind". The person who is being flattered should be indicated with the particle に (ni). 

Examples:

じょうしにごまをする。  Jōshi ni goma o suru. ("Jōshi" means boss)

I flatter my boss / bossflatter

せんせいにごまをする。  Sensei ni goma o suru. ("Sensei" means teacher)

I flatter my teacher / teacherflatter

Japanese sentences omit the subject when the subject is recognizable from the context. The implied subject is often inferred as "I" or "it" when there is no context.

When you want to specify the subject, use the topic particle は (wa) and put this at the beginning of the sentence.

かれは じょうしに ゴマをする。  Kare wa jōshi ni goma o suru. ("kare" means "he")

He flatters his boss.

マイクは せんせいに ゴマをする。 Maiku wa sensei ni goma o suru. ("Maiku" means "Mike")

Mike flatters his teacher.

Have you ever ground sesame seeds? 😆

Source : Idiom Dictionary with Pictures (Gakken, 2019)