Само собой.

Literally, it means something like by itself or of itself.

But as a fixed expression, it usually means of course, naturally, goes without saying, or that’s obvious. So the literal meaning is not the one you usually translate word-for-word in normal conversation.

It is a complete and very common expression.

Historically and semantically, it is closely related to the longer phrase Само собой разумеется, which means it goes without saying or it is understood naturally. In modern speech, people often shorten it to just Само собой.

So even though it may look incomplete to a learner, native speakers understand it as a full idiomatic reply or comment.

Само is the neuter form of сам.

In this expression, the phrase is impersonal. It does not refer to a specific masculine or feminine noun. Russian often uses the neuter in set expressions when the idea is general or abstract.

So you should learn Само собой as a fixed phrase, not as something you usually change for gender.

Собой is the instrumental case form of себя.

You may already know себя as the reflexive pronoun oneself / yourself / himself / herself. Its forms change by case, and собой is one of those forms.

In this idiom, the exact grammar is less important than the fact that Само собой functions as a set phrase. Still, knowing that собой is instrumental can help you recognize it elsewhere.

It is generally neutral-colloquial.

That means:

• very common in everyday speech
• natural in conversation
• acceptable in most informal and semi-neutral contexts

It is not slang, but it is a bit more conversational than something very formal. In formal writing, the fuller expression Само собой разумеется may sound more polished.

Both can mean of course, but they are not identical.

• Конечно is the most straightforward, general word for of course.
• Само собой often has a stronger sense of that’s obvious or that goes without saying.

Compare the feeling:

• Ты придёшь? — Конечно.
  Will you come? — Of course.

• Ты поможешь? — Само собой.
  Will you help? — Naturally / Of course, obviously.

So Само собой can sound a little more emphatic or idiomatic than конечно.

Usually no, but it depends on tone.

In a friendly tone, it sounds warm and natural:

• Поможешь? — Само собой.
  Will you help? — Of course.

But if said sharply, it can sound like:

• Well obviously
• Isn’t that clear?

So the words themselves are not rude, but intonation can make them sound impatient or dismissive.

Yes. Very often it stands alone as a full reply.

For example:

• Ты с нами? — Само собой.
• Можно? — Само собой.

This is one of its most common uses: a short, natural response meaning of course or naturally.

Yes.

For example:

• Само собой, я согласен.
  Of course, I agree.

• Это, само собой, нелегко.
  This is, naturally, not easy.

In longer sentences, it often works like naturally, of course, or obviously.

It is pronounced approximately:

suh-MOH suh-BOY

The stress is:

• само́
• собо́й

So both words are stressed on the second syllable.

They are very closely related, but not always identical in feel.

• Само собой = shorter, more conversational
• Само собой разумеется = fuller, slightly more formal, often translated as it goes without saying

In many situations, they express the same idea. The shorter version is especially common in speech.

Usually no.

Even though the literal meaning is something like by itself, the natural English translation depends on context:

• of course
• naturally
• obviously
• goes without saying

So when you see Само собой, it is better to think of it as an idiom rather than translate each word separately.