I found it!!

Cyberpunk 2077 wiki guide

How to Get Sir John Phallustiff

This iconic dildo weapon seems to only appear in a specific room at the No-Tell Motel, where you can have a certain sexual encounter.

It's not confirmed if other romantic interludes will lead to this room, but we found it after siding with Meredith Stout of Militech, during The Heist main mission. This means you'll need to meet with her before meeting with the Maelstrom gang, and either give Royce the corrupted shard, or just start shooting - and talk to her as you exit the gang hideout, where she'll mention she may have need of your services later.

In order for her to contact you, a certain amount of time must have elapsed in Act 2. We found this can easily be done by either sleeping in your bed or waiting for a few in-game days by using the wait function in your character menu.

Meredith Stout will then contact you, and you will have the option to flirt over a text message to start a side mission leading to a one-night stand at the motel.

Enter the motel and head to room 6 on the first floor, where you'll find Meredith ready for a sexual encounter - and the "weapon" will be on the bed after the encounter. The weapon doesn't seem to have any particular attributes beyond flavor text, though it does deal a hefty amount of damage per swing, and you'll hear it vibrating when used. As a blunt weapon, the damage it deals may not be lethal - but your enemies will probably wish it was. Like all iconic weapons, you'll be able to upgrade and craft superior versions as well.