Assalaamu alaykum. I have 2 questions:
1. If I hear someone give the Salaam greeting to another person (not me), do I have to answer it? For instance, if I hear a stranger say the Salaam to a person beside him (not me), and I am nearby, should I answer it too (in case the person did not answer his Salaam)?
2. Is saying "jeez" sinful? As we know, the word is a contraction of Jesus, whom the Muslims know as Prophet ‘Eesa, may Allaah exalt his mention. We use it as an exclamation or expression rather than as actually calling him, and nowadays the non-Christian also uses it. Is it sinful?
May Allaah bless you.

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.

Regarding the first question, the person who is not addressed with the greeting of Salaam is not obliged to return it, and if he did do so, then that is not sufficient in terms of clearing the liability of the addressed person from the obligation of returning the greeting. The Hanbali scholar Ar-Ruhaybaani wrote, "If someone else returns the greeting other than the greeted one, the latter still bears the sin for neglecting the obligation of returning the greeting even if he was in a group of people, because the greeting was addressed to him in particular." [Mataalib Uli An-Nuha]

Regarding the second question, if that word is meant to refer to ‘Eesa (Jesus), may Allah exalt his mention, then it is well-known that the Christians worship him. Their usage of the word may be unacceptable by the Shariah, such as when using it in invocation or seeking help from him, so using this word should be avoided. The fact that you use it as an exclamation does not make its use allowed. Allah, The Exalted, deemed it prohibited for the Muslims to say the word "Raa‘ina" (which means: listen to us or give us attention) in order to block the means that lead to unfavorable results (since this word was an insult in the language of Jews) even if their intention was correct. Allah, The Exalted, says (what means): {O you who have believed, say not (to the Messenger of Allah), "Raa‘ina" but say, "Unthurna" and listen. And for the disbelievers is a painful punishment.} [Quran 2:104]

Al-Qurtubi wrote:

"Ibn ‘Abbaas, may Allah be pleased with him, narrates that the Muslims used to say to the Prophet, sallallahu ‘alayhi wa sallam, 'Raa‘ina' as a request and desire for his attention. However, in the language of Jews, this word was an insult, meaning: 'hear us, may you never hear!' ... The Jews found it an opportunity to misuse this word and started to say among themselves: we used to insult the Prophet in private, but now we will do so in open gatherings, so they started to say to the Prophet 'Raa‘ina' and then laughed about it. Therefore, the verse was revealed (i.e. not to say Raa‘ina but say Unthurna [which means: Look upon us/Regard us]) so that the Jews would not follow their example and use this word intending its bad meaning." [Tafseer Al-Qurtubi]

Siddeeq Hasan Khaan wrote in his commentary on this verse, "This is proof that the expressions that may be interpreted as an insult or defamation should be avoided even if the speaker did not intend insult and defamation in order to block the means that lead to unfavorable results and avoid potential evils." [Fat-h Al-Bayaan]

Allah knows best.