Is the word sand a combination of the words sea and land? Well - I suppose it could be! But there’s no evidence for that.
Whenever I want to know about a word, its meaning and its history I always go to what I think is the most authoritative source - the OED - the Oxford English Dictionary.
(And now with the internet I don’t have to buy the 20 volume set at £750 because with my public library card I get a subscription free!)
According to the OED the word sand (used as a noun) means
‘ A material consisting of comminuted fragments and water-worn particles of rocks (mainly silicious) finer than those of which gravel is composed; often spec. as the material of a beach, desert, or the bed of a river or sea.’
Here are a couple of interesting facts.
The first reference to the word sand can be traced back to about the year 1000 AD (or CE) when it was used by Ælfric of Eynsham (c950–c1010)
who was the Benedictine abbot of Eynsham and a scholar.
I was surprised to find that there are more that 250 references in the OED about sand related words. Words like:
sand-spout n. a pillar of sand raised by a whirlwind in a desert.
sand dollar n. a flattened, irregular sea urchin belonging to the order Clypeastroida
sand-ball n. a kind of toilet soap
So now you know!