How to generate keypair completely offline?
Yes, it is possible to create such keypairs offline, but the calculations are long and complex. You would need to find out how to:

1) Create a secp256k1 public key from a private key

2) Perform SHA-256 hashing

3) Perform RIPEMD-160 hashing

4) Be able to convert the final value into Base 58

Generally, the calculations would be very tedious; this is what we have computers for.