Summary:

People are often very similar in appearance, behavior, emotional closeness, or act in sync with each other due to a strong bond, such as a shared ideology.

The reference could also be for entities that are closely connected in terms of traits, objectives, and activities, creating a strong bond of compatibility.

Article:

The pod, a protective casing, joins the peas together in one tidy package as they grow. The phrase 'peas in a pod' originated from the observation that peas, when they are still in the pod, are nearly identical in appearance, creating the image of similarity and closeness.

A pod is the natural home for the peas as they grow, encapsulating their shared journey. Most podscontain from a few to many peas, and we generally do not find dissimilar peas in a single pod. Other closely related terms are 'inseparable' and 'joined at the hip.'

The opposite, or antonym, is the term divergence, dissimilarity, difference, disagreement, sharp contrast, or sharply different.

Examples of peas in the pod:

My best friend and I are like two peas in a pod; We act, think, dress, and have the same daily schedule. Another example could be a married couple who finish each other's sentences and share the same hobbies and interests.

He is the brother I never had, like two peas in a pod.

Opposites:

My brother and I are opposites. Nothing between is similar. We do not agree on anything.