There are some things that are so normalised, so ingrained in our minds that we never think to question them. It's like that one cupboard in your house filled with books you've been seeing since you were born. Suddenly, 30 years later, you realise you don't know a single book in there.

The spelling of Thums Up is one such conundrum. There's an inexplicably missing 'b' in there, like the absent 'a' in Ajay Devgn. Except cold drinks aren't astrologically inclined... or are they? 

Source: Serious Eats

There have been many theories as to why the 'b' was dropped. It's been thought to be a mistake, a trademark bid, or just a unique Indianism.

Source: Livemint

One theory is that the circumference of the old 200ml bottles was too small to to fit 6 letters in one line of visibility. The pronunciation of 'thumbs up' and 'thums up' is more or less same, so they dropped the 'b'.

Source: Business Standard

Another theory is that it's named after a peak in Manmad Hills, Maharashtra that looks exactly like the logo. It's even known as Thums Up Dongar!

Source: Postoast

However, all of these have just been theories. 

The real reason is a lot more vanilla - Ramesh Chauhan, credited with building the Thums Up brand, said they had no particular reason for dropping the 'b', it was just a collective decision. 

About the events leading up to the creation of the name, he said, 

We were hesitant, because the name seemed to signal a sort of 'Up yours' and I didn't want it to become derogatory. I thought only westernised Indians, who were familiar with the gesture, would recognise it as a positive thing. We struggled a lot with that. But our ad agency leaned heavily in favour of the name, so we went with it.
Source: Marketing Mind

So there you go - there's no grand old story behind the missing letter. It was just a creative decision they agreed upon, and considering we're writing articles about it in 2020, it seems to have worked.