Screwing is street slang for having sex. It pertains to the visual similarity between penile insertion and what a screw does to a nut. This usage is considered crass and should be avoided. Likewise, calling “sexual intercourse” screwing takes away some of its sensuality.
More About Screwing
If screwing is the goal, then make sure to come prepared. It is not something that one would want to take lightly. There is an exchange in sexual energies between the parties involved. Likewise, there are inherent risks that come with screwing. Unplanned pregnancies may be the least of one's troubles when there is danger of contracting sexually transmitted diseases. Prepare for one's sexual encounter by bringing contraception and getting checked out for possible infection.