How to shorten script + what goes instead of or?

or is the correct term. This script is correct -- except:

You are using Part instead of part in all of the subsequence == expressions (variable names are case sensitive).

How or works:

• true or false is true
• true or true is true
• false or true is false
• false or false is false

And because or (ignoring execution order / short circuiting) is associative / commutative, you can just string them together:

A or B or C or D is the same as A or (B or (C or D))

Unfortunately there isn't a much shorter way to do this. We could use a hashtable (also called a dictionary):

local humanParts = {
	Head=true, Torso=true, ["Left arm"]=true,
	["Right arm"]=true, ["Left leg"]=true, ["Right leg"]=true
};

if humanParts[partName] then

Lua doesn't have a built in way to find objects in a table (which is sad).

If we implement one, though, then this condition is much shorter (but now we have a 7 line function definition -- that hopefully you find other uses for, but fair chance not):

function tableFind(list,value)
	for i, v in pairs(list) do
		if v == value then
			return i;
		end
	end
end

...

if tableFind(
	{"Head","Torso","Left arm","Right arm","Left leg","Right leg"}
	, partName) then

(I've changed part to partName in both examples because it is a more descriptive variable name; the variable is actually a string and not the object itself, so part is misleading. If you don't like the long name partName, then name is probably better than part)

A warning: mouse.Target can be nil (pointing at sky or, rarely, objects too far away). You should check that mouse.Target is an object before continuing, or line 03 in your snippet will error.