This is countering any verbal threat or violent physical attack, either with or without a weapon.
We need wisdom and tact regarding if we should engage and when not to. With personal snubs and offences the Bible’s teaching is not retaliation, but rather a “turning the other cheek” absorbing the insult in restraint and love (Mt 5:38-41; Rom 12:17-21). A quiet answer turns away wrath (Prov 15:1). However, Paul spoke up in his defence, informing the authorities that what they were about to do was illegal (Act 16:37-39). When teaching about the persistency of prayer, Jesus spoke of the widow who pleaded for justice; she would not let her enemy take advantage over her, but strenuously defended herself through the proper channels (Lk 18:1-8). We should not permit evil to go unchallenged for evil triumphs when good people do nothing. As believers we value life and so should protect and preserve it, yet not initiating any physicial confrontation.
Self-defence should be considered only as a last resort and be a carefully measured response to counteract any threat of physical violence or confiscation of freedom or personal goods. For example, the labourers rebuilding the walls of Jerusalem were armed to repel their enemies if they attacked (Neh 4:16-18). Where possible take precautions to prevent such a necessity occurring (Mt 24:43).
As believers we are under constant harassment in the spiritual realm and this is to be the focus of our proactive self-defence as we employ the spiritual armour God has made available for us (Eph 6:12).