I have found much trouble identifying the old word 'tis.

Does it mean "this is" or "it is?"

I have done some research and found that the dictionaries conflict.

One said 'tis :An old English word used in the place of this is ; a contraction of this is.

I showed this to my brother and he showed me another one it said 'tis :an old English contraction of the words it is.

as in:

'tis(this is) the voice of the lobster....or

'tis(it is) the voice of the lobster....

So how?