Translation guide
Describes the physiological state of having an erect penis. Japanese has several expressions ranging from clinical to slang, and the appropriate choice depends heavily on context and politeness level.
He has an erection.
Noun meaning 'erection'. Often used in compound phrases or medical descriptions.
Morning erections are a sign of health.
To talk about having an erection in very informal, often male-oriented settings.
Literally 'is standing'. A common euphemism among men. Context makes the meaning clear.
I've had an erection since morning.
Slang implying a very hard, rigid erection. Often used in a boastful or humorous way.
I got rock hard when I saw her.
Describes a state of being horny, which often implies having an erection. More about the feeling of arousal than the physical state itself.
I'm too horny to sleep.
To refer to an erection indirectly, often in mixed company or more polite contexts.
Literally 'is energetic/healthy'. A playful euphemism, often used for morning erections or when talking to a partner.
You're full of energy this morning. (implying an erection)
Literally 'is reacting'. A more indirect way to say the body is responding sexually.
My body is reacting. (implying an erection)
To describe an erection in a formal medical or scientific context.
The English phrase 'have an erection' does not translate directly into a single Japanese verb. Using エレクションを持つ is unnatural. Stick to the expressions above.
勃起 is the standard term and can be used in most situations, while 立つ is casual male slang. Use 勃起 when in doubt, but be aware that in very formal settings, even 勃起 might be too direct; euphemisms are preferred.